Hello Frances - I think from memory the ruler does have one diagonal line - I'll have a look in a bit - thanks for the tips!

If your ruler doesn't have a line from corner to corner you could mark it with tape and then just line up the diagonal lines with the lines from the pinwheel. Since the ruler is 8 and a half inches which is what you need for your unfinished block, just trim around the ruler.
Frances

I am doing some very simple 8" blocks - just a pinwheel. I have bought a creative grid square ruler which measures 8 1/2" square - not being the most logical person in the world - how do I trim the block down to get to 8" + seam allowance, keeping the centre of the pinwheel in the middle of the block? Sorry - trying to keep it simple - hope someone can help!