I was first inspired by Alternity's 3 levels of success and this was further reinforced by reading Steve Perrin's Quest Rules and by reading about some (proposed) changes to Call of Cthulhu's new edition.
So, I've given every skill 3 levels of success corresponding to "C", "B", and "A".
So If I have a Perception Skill of 45 it's broken down to 45 divided by 1, 45 divided by 2 and 45 divided by 5.
A "C" on a successful Perception Skill would be 45 or less.
A "B" on a successful Perception Skill would be 22 or less.
And an "A" on a successful Perception Skill would be 9 or less.
In an opposed skill challenge, whoever succeeds at the highest level wins. If the result is a tie, it goes to the "defender".