To avoid a long story, it took me several tries to properly adjust the y axis belts because when I removed the clip to adjust the belt, the belt slipped a little though the clip. Thus I no longer had a clue where it was. So the last time I marked the belt before the clip with a white grease pencil before removing the clip and when it slipped I knew where is was and advanced it 2 cogs. The pencil mark will never enter the axis assembly so should be no harm. I’ve read several topics which resulted in y belts needing tightening for noobies and as one I thought this might help.
I haven’t reviewed the latest assembly manual/document in awhile, however what I found to be an excellent informational nugget regarding X/Y belt tension in an early document was to mount one belt tension bracket flush, then thread the belt through the unmounted bracket such that it sat 0.25" away from the opposite side, then cinch said bracket up flush.
I used the method Jim mentioned and that seemed to provide good tension for me also.