So you're looking for the "magic ratio" so you won't need a tensioner. I don't know how to calculate it, but I know how to make an educated guess.

Each additional chainring or cog tooth will take up about 1/4" of chain. Trial fit your wheel and figure out the closest place you can join your chain. Estimate how much too long or too short your chain is and acquire a chainring or freewheel that has the right number of teeth to fit.