Math time: A 512 engine has 1049cc swept volume. To get to 10.5:1 compression divide swept volume by 9.5. That's 110cc.
ASSUME: Piston at zero deck, 12cc head gasket, 5cc valve pockets, 84cc head = 101cc. That's going to get you 11.4:1.

You need to lose 9cc from the half of the piston not under the squish area. Let's say you use a 3.5" circle, you need a half circle 0.0115" deep. That's with all my other assumptions.

It's just math.

What I think you and your engine builder are missing is that you don't have to buy the crank, rods and pistons as a kit. If you buy the pistons separately, you can get exactly what you need. Buying a piston with too large a dish and then milling a brand new head to get the compression ratio you need is IMHO pretty dumb.
Hogging out the combustion chambers to 93cc is also dumb, that's quite a bit of aluminum and you don't want to lose the squish pad area. So that makes the amount taken out of half the chamber pretty big.

R.