I would tackle it from a different angle since you are having a hard time visualizing what you need to do. You are making HST's from your strip sets so cut out a piece of paper the size that you want your block to be plus 7/8 inch for a seam allowance. Cut it on the diagonal(I would draw the strip lines on in the direction that I want them to be just to make it idiot proof) and use that as a template or measure the paper.
I mess up on mathing out things all the time so I try to make a visual safety check for myself.