i would check to see if the dinging is coming from the rear pulley being over-travel and binding up.
the contra pulley has just slightly more travel than the front pulley, so if the front pulley goes too far, it will put too much pull in the belt and a thrust on the crankshaft and driveshaft (trans input shaft) which is bad bad bad.
the 12, 10 mm measurement i'm guessing is based on my blog or a similar account of how to notch variators.
if you cut past that, you'll end up with the problem as described, weights travelling too much. I don't know if its better to have the window big and counteract with washers, or cut the window smaller, i haven't tried the 'harvey' method with washers to compare. sounds like folks are running it and it is satisfactory.