Take one of your braids and lay it on your cutting mat. Orient it as if the block were finished, using the lines on your cutting board to get it diagonal. Now take one of your rulers and lay it along perpendicular (up-down) edge as if it was one side of the finished block. Take another ruler (or anything straight) and lay it on the horizontal edge--so the two come together at a corner. What is the measurement on the ruler from the edge of the braid on the perpendicular side to the corner where the two rulers meet?
Add an inch to that measurement, cut the square that size and then cut diagonally. Trim if necessary. Try it with a piece of paper first so that you don't waste more fabric.
Oh, and it is a beautiful pattern!