The object is to get them all the same size. What I do, is take your block & ruler and measure from the same point on all 4 sides. 1st side go to the edge of block and trim straight across, then til you have a straight edge, next go to a seam on your block ,( I would use seam of 1st block on cross/t) bottom block is your red/green .Take a measurement from seam to top of edge you just trimmed, then do the same measurement on the other 3 sides. Hopefully you will get it straight and correct, maybe not 7 1/2 , just be sure you do the same for all blocks.