I agree with everyone else - start your measuring on the center seam of your block and measure out half way to what is supposed to be the measurement of your block. Another thing to keep in mind - all rulers are not created equal! Use at least the same brand ruler for your squaring up as you do for constructing your blocks. Rarely do I need to 'square up' a block if I've done my measuring/cutting/sewing accurately all along the way. Honestly, that doesn't happen all that frequently! I might have to do some stretching/squishing along the way. The only units I always have to square up/trim are HST's because I can't be bothered to measure 7/8" inch! I always just cut the whole inch size and trim down using the center line on the unit and trim all 4 sides. And I use the same ruler that I've used during the process. It does make a difference. At least, in my opinion.