I probably make more work for myself, but here's what I do....I lay out my quilt top on the largest flat surface available. I have only used regular 44-45" wide fabric - not wide backing. I then take my backing fabric and lay it on top of the top. I will fold the backing in half to see that I have enough length (and usually have more than enough). The fold the quilt in half. Does my backing show? By how much? If not, unfold everything and then do in thirds. Keep folding and unfolding until the proportions work out correctly. I'll then usually grab a pair of scissors and cut the pieces as they lay so I don't 'mis-remember' how it's supposed to be folded/sewn. So far this process has worked well for me and is far less taxing on my brain than doing calculations!