I see that ticket now, thank you. Good point I didn’t even realize that before, it looks like our PN just adds the “GT” specification into the middle of the generic 310-5M-15 PN so they are the same belt. We’ll still get another one sent out to you but that’s good to know.
What I usually do when tensioning is get the bottom four mounting screws just slightly loose, then pull back on the motor with one hand while tightening the screws with the other. Then I’ll leave the spindle cover off and run the spindle through a few different RPMs to see how smoothly it runs.
For tricky spindles, I’ve also started the spindle at 100RPM while the bolts are still loose so that way I can see how the belt reacts with too much or too little tension and try to tighten the bolts at the sweet spot.