Grog

Hey guys, I'm looking at getting a new set of rims 18x9/10's, but I want to make sure they're going to work on my '99 GT. The wheel specs are Rim size-18x9 and 18x10 offest (The 18x9's have a +12mm offset and the 18x10's have a +30mm) bolt pattern-5x4.5. What I'm not real sure about is the +12mm and +30mm offsets. My car has the eibach pro springs as well (which dropped the car about an inch). Any help would be greatly appreciated as I don't want to make an expensive mistake.
THANKS!!!!!!!!