Take it out in the sun (infinite distance light source sort of) and burn a few ants off. Use a tape measure or mount it on a rod. You are only trying to ensure it is 'about' 700mm, not measure it accurately. The focusser should take care of the rest. You can always fake the scope up on a piece of wood before hand as well to geta better idea of how it will assemble.
I always make the tube\OTA too long. Easier to cut a bit off than glue a bit on. The other method is to make a cell to hold the lens that fits(slides) inside the tube and adjust to suit then fix.
I have a 2" focusser and a suitable tube for a 100mm ..... but no suitable lens .
One day it will come together ....