You're overthinking the chain tension issue. The chain falling off is a function of chainline, not tension. If you have the rear cog spaced right (which you shouldn't have any trouble with, using a freehub and a spacer kit) then it'll stay on there. There's not a good way to calculate the links you need because distance from the center of the bottom bracket to the center of the rear axle is not constant. However, you really don't need to figure this out ahead of time. Get the chainring and cog that give you the ratio you want, then wrap the chain around the two. It will be substantially longer than you need- take it down to as few as possible with it still reaching.

If you were going fixed it would be more critical because play in the driveline is really annoying on a fixed gear. Not so on a singlespeed. Both my housemates ride singlespeed mountain bikes and both have fairly loose chains. It's not a problem.