If you have the crank, the BB and the frame, just try to put them together and measure the distance from the middle of the frame to the center of the chainwheel.
Also keep in mind that there has to be clearance between the chainring and the frame--this often limits how much shorter the BB can get.
You can also always fiddle with the chain line by adjusting stuff at the rear wheel, with spacers and stuff.
(It's good to finally see a bike topic in this forum. in between the flame wars and fashion advice.)