are you speaking of the "Hanging Hair Test", or of hair on your head?

If you are speaking of getting a "Haircut" with a razor you can VERY CAREFULLY lay the razor across a comb at the desired hair length and pull both comb and razor through hair.

please note, there are tools specifically made for this with safety guards to keep you from cutting yourself. While i would feel semicomfortable using this technique on someone else where i could see everything, I would not use it on my own hair as it would be easy to slip, and lacerations to the scalp bleed like a mother......

if you are speaking of getting the razor to pass the hht (hanging hair test) you will have to speak to those more knowledgeable than me