Formula to shave pistons to reduce compression ??

Have a shop mill a "D" shaped dish into the pistons, doing some very crude math, I guessed .250 in deep and dish dia of 3.28 in, I did not calc filet radius , I came up with a 17cc dish, between that and the appropriate gasket should get you where you want to go ........people with sharper pencils may do better

After exploring this though, you may find new pistons just might be easier