[Cam (Cam)]

I purchased mounted 127mm 700mm glass from Surplus shed. Had a mate who had a tube from an old reflector that will fit if I machine the threads off the cell. Im starting off with a focuser I got for free that came off a 60mm Meade refractor to test the glass. If its ok I will spring for a moonlite or sim. What Im worried about is cutting the tube length. Where do I have the focuser tube when measuring? in? out? half way? Do I allow for the diagonal? if so how much? Hope someone can take the time to advise. Thanks Cam