I was just thinking about this, and 5% per Margin of Recoil is probably the easiest to calculate and sanest.
The less sane option that I thought of was making it the square of the percentage, like so:
Code:
MoRcl %Hit
1 1
2 4
3 9
4 16
5 25
6 36
7 49
8 64
9 81
10 100
So there's a nice converge in how much you hit as you make an overwhelming better shot, and its possible for a glancing hit to only hit with a few shots.