I always have a bolt of inexpensive muslin on hand that I use to make patterns or experiment with things. This is one place where I would use that to try adding 8", subtracting etc. Once I had it right then I could use that as a pattern to cut out my nice fabric. I never use the muslin in the finished product as it isn't good enough but it is perfect for making patterns. I can write on it and it sticks to the fabric so few pins if any are needed.