Measure from the center of your crank bolt to the center of the vertical drop, but don't go along the chainstay. Measure to an imaginary point that extends from the dropout s if your ruler was parallel to the chain. Did that make sense? Then input all your measurements into the gear calculator software and away you go. Like I said, if you're going to try to do this without a chain tensioner you may have to get a halflink, which woud then require a 1/8 chain. I think Spicer Cycles makes a half link for 3/32 chains though...have fun!