When squaring blocks, I start with a middle seam and measure out equally on all 4 sides. Particularly with HST's. I would think that this process would work with your blocks as well. I would place the 45 degree line on my ruler to line up with the 2 points of your blue strip in the middle and measure from there. You'll need to rotate your blocks but then they should be even on all sides. Make sure your press everything well before starting.