if you don't want to mess with a chain tensioner, you can find that "magic ratio" using this tool: http://eehouse.org/fixin/formfmu.php

for example, my bike has a 415mm chainstay with vertical dropouts. i currently have a 32x18 ratio and i cannot get it tight, even with a half-link. according to that tool, i can use a 32x17 and it should be just about perfect.