Reference for serpentine belts

I did my own setup without P/S or A/C without using any of expensive idler pulleys that bolt in place of the pump or compressor. I ended up just using a string to guesstimate the belt length and took that to the part store and asked them what they had in that length. The one I got was a little on the too short/long (don't remember which), so I just took it back and asked for the next size they had. Been working fine since then as far as I can tell.