I read some where that rating lift heights shouldnt be based on one number. And that it should be based on a set of numbers. And i would have to completely agree.
a set of numbers that should be important should be angles of approach and departure, ground to lowest point under the jeep, and ground to axle. numbers like that mean a lot more which im sure you are well aware.
Sorry i didnt help answer your question.
I think maybe the difference between stock wheel center to a point on the fender and the center of the wheel and the same spot on the fender with the lift.
the first picture has to do with springs, and the second is two screen shots i took from roughcountrys youtube video on their 4 inch lift. Hope these help.