Interestingly enough, I asked this same question on the AMCA forum and got answers similar to Carl-Eriks, 31"...or 787mm. My fender actually measures slightly less than 31", so the arc of my fender is not the problem!
What is interesting is that there was a suggestion that perhaps this fender is from a 4 cylinder as those bikes had longer forks! Apparently the fenders are the same for Scout/Four.
I've got to "mull this one over" !