AFter I make blocks, I like to square them all up to the exact same size using a template and a rotary cutter around the edges. Then I know if the sash is the same size, and the blocks are the same size, it works better. My best bet is to square all the blocks to the exact same size. Just a 1/8 of an inch on each blocks makes a huge difference in the end. Good luck. We've all been there, and some of us still travel that trail now and then.