... So the air inside has center of the geometric center of the tank interior. You could likely get a rough approximation with a picture of the tank.
Yes, probably not a big deal. Problem is that I balance the tanks with the regsets attached so the center of mass of the gas won't align with the balance point. I need to test the swing in balance point.
... And does the center of BC lift correspond to the center of tank air that you are lifting with it?
No, but they are connected in an exact way by the backplate.
... Finding your center is the first crucial step.
Yes, you obviously understand what I'm doing. The slots on the rack that attaches to my BP are on 1 inch centers. Thus, I'm at most a half inch off on my balance point. I don't need more precision than that for my zero. I get into frog position and extend my arms, hands clasped. I next retract my arms. I then seek the most "natural" or comfortable position to place my hands. Legs in what I think is perfect frog position. The GoPro is the after-the-fact judge of that.
Here is where I will speak to "skills". I am absolutely convinced that 80% of "skills", so often referred to, is simply being able to alter leg and arm position to make up for imperfect gear balance. How about we start with correct gear balance and THEN add skills?
... How accurate did you find you could get that or that it needed to be? Roughly how much shift in it did you see for frog vs flutter?
A lot. Heavy fins and ankle weights make a big difference. Remember that you have two things to consider, neutral buoyancy and trim.
Buoyancy is just adding up all the scalar vertical forces (float, sink) and figuring out the best way to deal with the deviation from zero.
Trim is found by making all the moments of rotation cancel. Simple vector sum. Again look at the deviation from zero and adjust placement of weights. Rotational force of anything (stuff you add to "the basic you" like a can light) is its U/W weight times the distance from horizontal trim zero (marked on BP). Keep the signs straight, pick your own convention. I use positive as rotating my head upward, negative for forcing me head-down. Bookkeeping.
You can have perfect buoyancy and be diving a balanced rig (Hogarthian meaning of balanced) and still have horrid trim. That was me for years. The trick is to align all three.