I frequently make my blocks oversized (I blame Eleanor Burns, but I like a margin of safety) so I always have to square up. I measure from the middle out -- if you look at Melanie's block above, I would make sure the center of the 4 patch is the same distance from each edge so I would mark not only the desired size but also the center of the block on the ruler (I use glow line tape) so I have what should be in the center exactly there.