Sometimes you just need to have a crutch! Decide how long your rails will be and then cut your fabric 'almost' in half. (Directly in half may cause too much waste, so maybe an inch or two in one direction will be better. Example - finishes block is 6", so the set will be cut in 6 1/2" blocks, therefore I split my WOF in 20" and the leftover. I know I will get a full 3 blocks from first and only have waste on second part.) Then work with the smaller strip sets. An extra benefit is that you can make more set combinations this way.