Now, as a consumer having just bought replacment belts, I feel challenged to make each one last at least several thousand miles!
Moving washers from one side to the other has NOT stopped the clicking sound on belt/spoke ring contact, but is has reduced to a barely noticable level. More of an assurance that the tension is good, I went to the Whiteheads to compare my new frame-mounted tensions to the two bikes in their barn, which are axle-mounted, and its VERY close to identical.
When you have the install done, belt on, spin the rear tire around, checking clearance to the frame, that the belt is not rubbing TOO much (or not at all) on the drive ring walls, is a centered/cradled as good as you can, you take your first test ride !!
Just a half a mile say (usually I have to come back anyway and re-adjust the seat), but LOOK AT THE BELT, see if one single thread has frayed.
NONE? Good.
ONE? Okay, whichever side the fray appears is the side that needs one washer to shove the wheel the other way. Take a lighter and burn off that frayed thread, so you can retest on a longer ride.
First LONG 5 mile test, look at the belt again. Absence of frays is perfection of the install.
IF the belt breaks within an hour of letting a friend ride your bike,
that is the most likely culprit.
Strangers won't understand the techniques you've established climbing hills, but the engine really gets used to one persons throttling habits, and not helping on steep grades is usually the
second worst thing for a belt slipping, next to having a spokering way out of balance/too much tension arm jumping.