Lots of options to explore:

the still point is "within" each experience, in it's center, which is empty. so if you experience anger and go to the center of the anger, you'll find it isn't "angry" in there. you'll find that if you go to the center of the feeling of pressure on your butt when you sit, there is no pressure in the center. etc.

the who that watches is in the visual field, vaguely associated with the location of our eyes. if you poke your finger toward your eyes, the place you are aiming at is the watcher

the who of the watcher is in the sense of "attention " that moves around to objects within awareness. it is "in" what is watched. you might be aware of the whole room, but you have one place/thing in attention, maybe these words, and "I" am seeing these words is a felt-experience... and it can feel like the "I" is actually in the words.  the who can be in this moving around "I-ness". the moving around I-ness can be very fast and precise during some sits/investigations, so not a still center, but a dynamic center that doesn't change it's essential "i-ness".

when ever you mostly calm down, but feel a sense of natural vulnerability or core sense of lack, that can often locate at the heart. it can feel very personal, very essential. I am vulnerable, I am imperfect, I am inadequate, I don't understand awakening... or even the reverse: I know who I am, I am powerful, I am knowing and intelligence, I am in control... these sensations are very often located right at the heart center. Many times when you say "I love you" and you point somewhere, you point to the heart. there are many practices that serve to awaken and make tender the heart area. classically "metta" is like this. so is "taking and sending". actually you can explore how the heart responds to all of the different meditation practices, it's very interesting. It clearly shows that greed, aversion, and indifference truly are poisons.

It does seem that when the knot of self releases, the knot was located at the heart. But be gentle with this idea. Sometimes people want to force heart openings and wind up damaging themselves. The person has to be good and kind and aware...THEN the heart softens and opens up, on its own, like petals of a flower opening in spring. Never try to force open a flower by pulling back the petals.

Hope this gives you some ideas to explore!  In general, I would explore them in this order, but have fun with it, choose your own approach based on what feels right, and make the exploration your own!

This question came to me on its own during sits or off cushion each time there was attention (noticing) on the seeing-nose-behind eyes-watcher. My Noting would be freestyle, noting anything that is. Then there was this noting "who is seeing, who is feeling" then it would go back to other sense events just as they are without wondering who is experiencing. This comes and goes in waves. I find it's good to let it wave on its own but keep attention (noting/noticing) continuous rather than getting lost in a narrative of "is it in the center of this experience or is it in the heart, maybe in solar plexus)  

I would suggest keep the stream of attention/noting constant and unbroken and let questions on who is this and other sensate experience dance their dance in the light of awakeness. 

just my two cents. 

   The heart is on the left side of the middle of the chest. Bra.

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Don't want to hijack your topic but I frequently come up aggainst these kind of questions in my practice too and get stuck:

• As you gaze into space, very gently and briefly, look for what's looking. Look for the center of consciousness. Is there something to find? Can you find your head? Or is this just this open field in which all visual perceptions are appearing?" I have trouble with these. I’ll ask “Do I feel something?” “Yes, it’s in my head”, “Where?” “Between the eyes”, “now where?” “can’t go any deeper, there is no thinker/observer/self.” But I don’t really feel the process of looking back. Could you please help with this?

 
The energetic heart center is just where Mike Smirnoff described, just a little to the right of the middle of the chest. 

Don't want to hijack your topic but I frequently come up aggainst these kind of questions in my practice too and get stuck:

• As you gaze into space, very gently and briefly, look for what's looking. Look for the center of consciousness. Is there something to find? Can you find your head? Or is this just this open field in which all visual perceptions are appearing?" I have trouble with these. I’ll ask “Do I feel something?” “Yes, it’s in my head”, “Where?” “Between the eyes”, “now where?” “can’t go any deeper, there is no thinker/observer/self.” But I don’t really feel the process of looking back. Could you please help with this?

Maybe in reality there is no "looking back", just some sensations in our heads (a small fraction of your nose and eyes and part of body) and awareness (visualisation of bending, arching back) dressed up as a watcher seemingly watching other sensations? Everything on "one side" of one sensate field.

lol

I am not sure I understand. Can you explain this further?

Just some quotes from - I think - Daniel's chapter on formations about awareness being yet another cluster of sensations:

• 1) the sense of the observer on “this” side; 2) most of the sensations of the body; and 3) most of the sensations of the mind, with the latter two fluxing “over there”.
• As mental and physical phenomena gradually integrate with the sense of luminous space, this experientially begs the question, “What is observing formations?” As formations become predominant, we are faced first with the questions of which side of the dualistic split we are on and then of what is watching that which earlier appeared to be both sides. Just keep investigating in a natural and matter-of-fact way. Let this profound dance unfold. If you have gotten to this point, you are extraordinarily close and need to do very little but relax and be gently curious about your experience. They reveal a way out of the paradox of duality, the maddening sense that “this” is observing, controlling, subject to, separated from, etc., “that”. By containing all or nearly all the sensations comprising one moment in a very integrated way, they contain the necessary clarity to see through the three fundamental illusions.
• One of the primary ways that the illusion of duality is maintained is that the mind partially “blinks out” perception of (a part of each formation that it wants to section off to appear separate) (ja: this what seems to be self), in control, or observing everything else. In this way, there is not enough clarity to see the interconnectedness and true nature of that part partially blinked out of reality, and a sense of a separate or autonomous self is maintained. The problem is not the arising of those sensations and patterns that the mind is partially blinking out to, it is that these (self)sensations are not clearly perceived. It is almost as if the mind is placing some sensations in its map of space and what is in it, and then only partially doing that for other sensations that it wishes to turn into a sense of something stable and continuous. When the experience of formations occurs, it comes out of a level of clarity that is so complete that “blinking out” can no longer easily happen, as everything is mapped equally and completely to the same volume of our perceived sensate space and so, finally, the clear perception we have developed threatens the core illusion of a stable, perceiving, separate self. Yay!
• In practice, we gently work to integrate those subtle remaining processes that seem to be a “this” side, a practitioner, a practice, a self, and so on, into the rest of the clearly perceived sense field, basically just by being gently aware of the patterns of sensations that make up those qualities of space until they realize they are naturally aware of themselves and the thing flips over.