A 416 has about 853cc swept volume per cylinder. To get r compression ratio, one divides the swept volume by (r-1).
So to get 12.5:1 you divide 853 by 11.5 = 74.2cc above the piston. For 10.5:1 it's 853/9.5 = 89.8cc

So you can see you need a way to subtract 15.6 cc. BUT, you still haven't told us about block deck height, head volume, etc.

Just saying a piston is "12.5:1" doesn't tell us much.

R.