I do not do math, but I need to know if there is a formula for figuring batting/backing needs. I usually just buy what fabric I THINK I need but that doesn't always work. I will be teaching a D9P class where some students will be making table toppers, others will be making runners-with & without borders-so nothing will be the same size. How can I tell the students how much backing they will need? Is there a math formula that will cut down on the amount of stash created from buying too much? Thanks for all your guidance.