In my opinion it is too much trouble to add or decrease numbers from D20 to-hit roll so it affects AC. It's easier that way.
- Partial Cover 25%: AC -2
- Half Cover 50%: AC -4
- Full Cover 75%+: AC -6
Partial cover might be behind a rock waist down.
Half cover might be behind a window shooting.
Full cover might be laying low behind a bush.