I just do it till it looks and feels about right, don’t really know what right is but not too loose and not solid. I did recently have a problem with mine though (again 4.0 zj), just next to the alternator there’s a plastic idler wheel, when the belts off just check there’s not too much play in it as mine broke coming back from jeepey jamboree.
The belt came off and went behind it and had stated cutting through it, it made it home and I didn’t realise till the day after when I was checking out what the funny sound was.
Alternatively I may have forgot to re tension it and it may have slid off and then broken the idler? Not sure but either way it broke.
