You really need to work out a few more numbers, as it also depends on the angle between the tubes of course. How are you designing your frame? If it's on layout paper, you can just draw the tubes on the paper and measure. If it's in BikeCAD or similar, the software can give mitre-to-mitre distances.

Other things to remember:

If it's butted tubing, then you're only meant to cut one end - ends are marked.
Mark the tube so you know which is the top and which is the bottom, so the mitres align.
Cut a little long, and gradually file away until it fits.