It depends on the species of snake, the condition of the slough, it probably varies between individuals and the sloughs are quite elastic. Try thoroughly moistening one and stretching it - you might be surprised. If you allow it to dry while stretched it will keep its length. The skin is (usually) fairly supple when it comes off and can potentially 'set' within a wide range of lengths after it dries. There is certainly no reliable, stable ratio, even for the same snake. A 10' snake might have sloughs between about 11' and close to 20'. The misconception that you can measure a snake from its slough is no doubt one of the reasons that the overwhelming majority of reports of snake lengths are larger than the actual length (other than for snakes under about 60cm, which are usually underestimates!).
Snakes are quite difficult to measure and I'd pluck 95% as a figure out of the air for the proportion of reports which aren't worth listening to. It certainly would be nice if sloughs could solve the problem!
