I did a block for a sampler quilt that is being auctioned off for breast cancer awareness.
I was freaking a little because I am so new to quilting and they needed it to be exact! The math is fairly simple when making a patch type block...but I agree with one poster who said build it up and then place a border around it. Then square it up. They wound up liking my block since some of them came back with points that went clear to end without leaving enough to sew other blocks on without messing up the points! Mine could be added easily.
Good luck with your project.