I measured it myself on jeeps. A 2.5 inch lift gives a measurement of about 2.5 inches but it depends on the weight of the Jeep such as V8 or 4 cylinder. I think it's easier and more accurate than measuring the fender height.
I assume you have measured stock springs as well?
you are the 1st person I have seen that has done it this way, I am in the same boat right now trying to determine stock height.
I started on this search after installing a 2" lift, Jeep is a 83 6cyl, (untouched before)
heck it didn't even raise the rear 1" at the body, I had no measurement for the front as the front springs were completely flat so I could only assume the rears were shot as well which should had given me for than 2" not less than 1". ( attached a image of my from diff bump stop clearance to show how collapsed front springs were)
I have since measured from top of axle to frame at bump stop front and rear and shock stud to shock stud front and rear,
during my search, I have been finding many different results from lift at the body too, most say stock is about 21.5" to bottom of tub, unreliable though if body bushings and everything else are worn,
but also from stock so is about any other measurement that does not have new factory springs on it, there is literally no factory spec except overall height, and step height (do they measure that at the tub opening?
I even contacted Jeep about it, I wonder how a body shop would ever check a frame for light damage then?
anyhow I am putting together a list of this information for future reference when I can find reliable information
BTW thanks for your time!