Here's the extract - with instruction on using an Omnigrid 98l, but also giving instructions to cut if you don't have a triangle cutting ruler (that's me) taking your ruler and marking with tape a line running across the corner, 6 1/2" along each edge. Handily, my Creative Grids ruler already has this line marked on (but I put this red tape line across anyway, for clarity).
Well, when I line up my tape edge with the bottom of the tube, the point pokes above the top end by about 1/4" - enough to make a triangle with a chopped off top.
And if I move the ruler so the point touches the top edge of the tube, the tape line is about 1/4" below the bottom edge of the fabric. (I've checked my seam work, and the 1/4" seams are accurate).
So to the crux of the matter: should the triangles should be just that - triangles - with the purple lines I've drawn with the air eraser pen zigzagging properly point-to-point all the way along the tube?
I'm afraid that you'll have to peer quite hard the pics to see the purple marker line (like all pics, click to enlarge, then backspace to return), but before I go cutting, I'd like to make sure that I'm cutting in the right place...
Joanna quite rightly points out that the excerpt at the top is the instructions for Omnigrid 98L users - so here is the extra bit of instruction which is relevant to non-Omnigrid users (me!).