I'm self taught Rigid Heddle Weaver too. I haven't tried winding on a tape measure, but I do use removable stitch markers (they look like orange and blue plastic safety pins made by Clover I think) to keep track of my length and keep a ruler or tape measure at hand. Once I knit 5 inches in length, I clip an orange removable marker on the selvedge of the work at that point to mark the first 5 inches. At the next 5 inch mark, I place a blue to mark 10 inches. I then alternate orange and blue. I sometimes keep a record of my accumulated length on scratch paper, sometimes I just make sure the markers protrude from the edge so I can count them. I then know how much I've knit. I don't always just mark 5 and 10 with the markers. Sometimes I just measure, mark, record length on scratch paper - and then when I advance I place another marker or just move the one I had and record the new length.
If I am doing a pattern, I do what Betty Davenport advises in one of her books. I use a strip of paper to make a template ad mark the distances between color changes so I can get a match on the other end.