Originally Posted by FBinNY

Simple math.

The hub moves forward roughly 1/8" per added tooth, likewise it moves back by the same amount with smaller sprockets.

Here's the reasoning.

Each tooth adds 1/2" to the circumference of the sprocket. The chain wraps half way so you need 1/4" more chain. That's divided between the upper and lower loops, so the hub has to move 1/8" to make up the difference.

So, measure the amount of usable room and make a decision. If you have the dough, you can reduce the chainring and increase the rear by the same # of teeth, and the hub will barely move. (you still get a bit of movement because the upper and lower loops aren't parallel, which screws up my simple calculation slightly)