When I did this first at a class, the instructor had us measure several different spots on the strip set and take the narrowest measurement. For instance if your strip set measured 6" in two places and 6 1/2" in another spot and maybe 5 3/4" in the narrowest spot, then we were to cut then strip set into 5 3/4" squares. What I always found was that my 6" strip sets were never that far off, just generally a scant amt...but I was always careful to use the lesser measurement to cut my squares. I imagine this is about as clear as mud, isn't it? Sorry, but it does make sense once you have done it.