The correct adjustment is what’s in your manual. Both how tight, and how to measure it.

The rest is opinions. Some well thought out, some not.

 

I have always adjusted my belt within specs. but on the loose end of the spec. Been working for 31 years.
I think the bigger issue is getting the belt straight within the swingarm. Center of swing arm pivot to center of axle and then set the tension. A belt running true and tensioned correctly will last a very long time.