Please return to the flat +4 Cover modifier. While I admit the varying cover seems logical, it's unnecessary. Here's why:
While minis on the table look so stationary, they're supposed to represent a lot of fluid combat motion. If someone is participating in combat at all, they have to come out of cover a certain amount to do so. Any difference in how much cover they have (so long as it's less than total) doesn't really matter. They're moving around and anyone attempting to attack them is assumed to be waiting for the most opportune moment to do so.
3.0 used variable cover rules. Ultimately they decided to go to flat cover in 3.5 for the reason listed above. It's simple and elegant, easy to remember, and really makes the most sense.