You can figure it out with feeler gauges. Your A,B measurements need to be made at the firing pin..

A: You need the distance between the frame and the heel of a .45acp when in the cylinder.

B: You need the distance from the cylinder to the frame with no case.

C: you need the thickness of a .45ar rim.

B-A=?

C-?= the amount you need to cut off....

Make sense?