oops, sorry, about the confusion. The good news is this is not a crucial math formula because if the elastic is a little too big, it’s easy to correct by overlapping it more. To be honest, when I’m doing it to myself, I just wrap the elastic around myself and cut it, maybe making it a little larger, just because it’s better to be a little too big than too small. When I’m ready to sew it for real, I do this again by overlapping the ends, pin them together and then try on again. For elastic to work, it needs to stretch to fit snugly, therefore, it will end up being a little smaller than your actual waist measurement. After it feels right, I’ll trim off any extra elastic that goes over my 1″ overlap. Hope that clears things up for you.