Tightening a cooler is done by feel. You tighten until you feel that you need to put real strength into it.
As in as long as you need to put a tiny bit of strength to keep tightening it, and all screws need equal amount of a bit of strength, you are ok. Once you feel that you need to work a bit harder, its time to stop.
And please don't jiggle the cooler to check if its tighten enough. You will just end up breaking the motherboard.
I might be a dick, but I'm not stupid.