There isn't anything wrong with the 5 cut method! Stuff like saw dust can throw off any method, 5 cut, square, anything else someone can think of. Care must be taken when performing the test, just like it should when you are doing any other finicky task where accuracy matters.
You don't even need special tools to use the 5 cut method, a $10 set of feeler gauges from the local hardware store will get you within 0.002"/ft.
If someone is familiar with the procedure, it will most likely take 2 tries. The first pass is so a measurement can be taken to see how far out the sled is, and the second is to verify the results after a correction is made.