Kami, you are right the balance point is where you can stand feet side by side.
But you are very wrong about the mast connection being the pivot point.
AndersP is right
On a stabless foil, the pivot point is around 30 % from the leading edge of the front wing.
On a normal foil it is maybe 10 cm further forward, close to the front of the wing.
Because the stab moves the system COE forward.
Also, the pivot point is under the water, at the very wing, not on the board, and never ever the mast.
It does not matter in this respect, where the mast is positioned, forward or rear of the wing, the pivot point is at the same spot on the flyer (flyer = frontwing+fuselage+stab)
The balance point on the board is pretty much just over the pivot point of the flyer, so around the front of the front wing usually, maybe a tad more rear when riding powered because of the wing drag and lever effect
I always have my rear foot behind the mast, as I don't like narrow stance.
If I put it on top of the mast, I would have a really narrow stance on most hydrofoils
But some like this, we are just different
Peter