I sew mine with a 3-step or regular zigzag in the seam allowance before I turn, but I don't measure the elastic first. I tack it down at one end and zigzag it on pulling it tight as I go, tack it down at the other end and cut. I would guesstimate 6" or less needed. Oh, and I use 3/8" polybraid. I don't care much for lastin for dipes.