Yeah LOL I hate that belt tension crap, it's either just loose enough to 'slap' around or it's too tight SOB if I ain't been down this road time and again, and it can also be just tight enough where it wears the spindle bearings prematurely or the belt breaks in 3-6 months because it was too tight and other parts wear as well...
Man...
I tell you how I do it, is I take my time, it gets a little easier every replacement but it still takes fiddling and twiddling to get it as close as possible and at least in my case it's never really perfect but such is life.
Roughly, starting out about as tight as an alternator belt, maybe an inch of play side to side... So, tight but not super tight, hard to explain it just takes practice.
Too tight is no good, I find if it's tight enough where it doesn't slap around at all then it's also too tight, ain't life grand.
I like it to be where it doesn't slap a lot, the biggest issue is I watch with the cover OFF to see if it's hitting any metal objects because if it is something has to change or it will wear prematurely. So then one either tightens it or loosens it, quarter turn at a time (fun), until ya get it somewhere between minimal slapping around and not too tight, close as you can get it to that.
Way I see things, I think you'll smoke a few, probably break one or two prematurely, then once you get 5-6 years and 3-7 belts down the road you should have it down pretty good

That's how it worked out for me, I still don't always get it right, but as a rule if it is right or close to it, I've gotten 3-4 years out of such a belt before (so don't listen to what the dealers tell you that 6 months is normal lol).
Peace out
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Other notes: On both of my mowers I removed that stupid spindle brake made of steel, the belt kept hitting it.
Now the spindles free wheel when shutting down, always have to wait a good 30-60 seconds for them to stop, but it beats having to replace the belt more often than it really should need it.