Originally Posted by Darth_Firebolt

for vertical dropouts, you want to find a magic gear. all you need to know is chainstay length (fairly accurately).
http://eehouse.org/fixin/formfmu.php

plug in the chainstay length, largest chainring and smallest chainring you'd want to run (smallest mostly depends on smallest BCD mounting points on your crank), the largest and smallest cog, and tire size. the line will show you gearing that will work with no tension devices. gear combos in the same color use the same number of chain links. pick something to the right of the vertical line on the chart, as the further to the right you go, the more slack is added. I wouldn't go more than 1-2 mm longer than your actual chainstay length or your chain will get too slack.