Fraz- The method described by zzt is correct. But, such a calculation may not be useful unless you know exactly how far under the targets you are shooting now.
Most of us take a different approach. If we are shooting too low, we raise the comb up 1/8 of an inch and see if that helps. You will need to shoot a lot of targets to determine if the 1/8 inch change made a positive difference. I do make some stock changes now and then. I know I probably shouldn't, but I do. I try to shoot around 1000 registered birds before I decide if the change was good. Once in a while, I only have to shoot 50 birds to realize that the change was bad.
Pat Ireland