There has been a lot of conversations about this for years, you are not just dealing with the aerodynamic centre you are also dealing with the centre of pressure, most RC stabs are symmetrical so the lift feedback is not involved.

The AC point is fixed and the CP moves along the surface towards the trailing edge as the lift increases on a non symmetrical airfoil.

This topic also brings in mass balance of the stab and again this is based on where the actual pivot point is. And again that topic has been talked to death for years.

There is normally a lot of math's involved with all the above and yet going slight forward of the MAC is normally sufficient for most RC applications.

Regards,