Well, factory measurements are 17" rear and 17.5" front to bottom fender flares. Would it be possible to find another XJ with uncut fenders, find a common point between your rigs, then make the measurement from that point on yours? Body seam, top of hood and window?
On my 98, stock, from the bottom of the fender flare to the body seam it is 5.5". On the rear from the bottom of the fender flare to the body seam it is 8.75". So if from the center of the wheel to the bottom of the fender flare is 17.5" stock front, then add 5.5"
23" stock front from center of wheel to body seam and 25.75" rear measurments.
My 97 has a 3" lift so it SHOULD measure out 26 front and 28.75 rear. I will confirm this when I get home from work.
Hope this helps out.