Ive used this twice now. It works perfect every time. However ive only used it when i have caught the girl looking at me from across the dancefloor.
I blatantly walk up and say;

Me- Hey, would it be wrong if i asked for your number?
Girl- No
Me- If i did ask for your number, would you give it to me?
(if the girls says no, then sweet you walk away or try tuning some more, but if she says yes)
Girl- Yes
Me- Well, then can i have your number.

This has worked perfectly for me each time.
So I'm unaware of any falws in it, but you guys should test it out and see what happens. but the key here is to be confident.