You must know the radius of the bend you are going to make.Then you figure the stretch factor of the tube.This will tell you how much tube is required just to go from tangent to tangent on the bend. Then you add in the length of the straights. I bend tubes for a living. I have the formula for figuring stretch factor at work. I am at home now.If you want it I can post it up later tonight on my lunch break.
tangent = where the straight stops and the bend starts