Please don't scrimp by sending them the exact length: They won't be able to use any of the tubing delivered.
Consider the physics (or mechanics of the machine doing the bending the square tubing. (Pipe is similar, but round. 8<)
One: First, an inlet straight part of the tube is clamped in the machine by either a vise, or forced against an anvil sized to the outside of the tube steel. Length of the inlet straight is determined by the shop's machine. For my wrought iron bending, I only need 3-6 inches. Commercial shops need 4-10 inches.
Two: The inside radius of the bend is determined by a mandrel at the centerline of the bend: OD/2 of the mandrel = IR of the bend. A second straight part of the tube must continue past the end tangent of the bend.
Three: The machine forces this excessive outlet (straight) tube around the mandrel by either rollers on the outside of the tube or a sliding second mandrel, levering the tube around the mandrel.
As pointed out above, the material in the tube itself yields both on the outside and inside of the tube, but NOT equally. Neural axis is about 5/16 (1/3) across the tube: you can see paint or mill scale flexing off of the inside surface as it compresses, and can see the outside wall of the tube fail and "bend" inward if the IR is too small. Cheap fabricators don't care about the outside failure, and sometimes do it deliberately to try to get as tight a radius as possible.
So even if you could exactly calculate the theoretical length, you'd need to trim both the inlet and outlet ends of the unbent tubing at an angle to make up for the different stretching distances across from inside to outside. If that were done, then they could not bend the tubing because the shop could not use either the inlet or outlet clamps on the tube bender to work as a lever to oppose the bending force. (Hot work by squeezing soft red-hot metal into a die is different, but then they still need a way to clamp the metal being forced into the die.)
In practice, sometimes the shop makes the bend on the first piece with a little bit of wastage, but then trims the bend so the excess at the inlet of part one become the outlet lever of part two, etc....
Call your customer and see how they can supply you with their scrap for recycling, and what lengths are most efficient to go through the production line. Perhaps you can save them money with longer (or shorter!) raw material lengths.