Those C-Spanners are not necessarily bike tools - I got mine from a general hardware store, I think the ‘ring’ could be called a ‘lock ring’ ?
Snubber gap to back of belt should be as small as possible without being in constant contact with the belt. 0.25mm would be about right 0.75 or 1mm too big. If you belt has been running with loose snubber and tension for some time (ie regularly popping) it may be possible you have damaged a few kevlar cords - reducing the belt’s stiffness and so making it more prone squeezing past snubber, and so jumping more than a new belt would do.
I find a THIN flat spanner is best the get to head of snubber between it and disc – OR a slim socket through the disc. Adjusting the snubber to get the be optimum 0.25mm gap is fiddly … I seem to need 3 hands !! (1/2" / 13mm Spanner for head, 0.25mm feeler gauge / plastic strip to set 0.25mm gap, and another 11mm spanner for the lock nut ).
If all goes well your belt will not ever jump - good luck (this message will self destruct in 3 minutes 
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