I go by feel.
If I can wiggle the belts (we're talking alt and PS belts) more than about an inch on the longest span, then it needs to be tightened. If you're worried about it being too tight, go with what feels good and start it up.
If it squeels, tighten it up. <img src="/forums/images/graemlins/oink.gif" alt="" /> <img src="/forums/images/graemlins/grin.gif" alt="" />
Hope this helps,
Todd

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car