Having been sternly warned of the difficulties of cutting fabric on the diagonal, and how the resultant triangles can go all stretchy if not handled with respect, I have invested 98p in a can of spray starch & have liberally applied to each side of the fabric tubes & pressed them with a dry iron them such that they are stiffer than the board.
I then took the ruler, measured the first triangle (quite a lot of times!), took a deep breath and sliced the fabric tube up to give eight triangles.
Then I had to go and have a sit down - there is a distinct lack of room for error here (unless I wish my garden trellis quilt to consist of 79 and not 80 small squares...)
I cut the triangles from the second tube, then very gingerly opened them out and pressed them flat.