I don't worry so much about the final measurement most of the time. I just square them up to the size of the smallest block. If my seams are a tad over and the block comes out a bit smaller, I just make up the difference with the borders if I need it to be a certain size when finished. That is one reason I seldom cut the borders when I cut the blocks out. Just in case I decide to adjust it a bit. No matter how perfect you cut, the variance in the fabric can affect the size too. Some fabrics have more give to them.