I have learned to use the diagonal 45* lines on the ruler to square up. If it is an 8" block, I follow the 45* line to 4" then mark with tape at the edge of the "center point" and draw a line on the tape to point out the exact center and use that for all my blocks as the center point. trim away the little bit on all 4 sides. If there are no seams to center the block and I know I have a 1/4 seam throughout then i would fold the block in half, then in half again to find the center (iron once folded and it is easy to see it.) Always always check first block.
Maybe it is possible to square the blocks down to say 7 1/2 inches, (I do not see the pattern) and use the blocks you have already to make a smaller version? or add sashing in between blocks.