Ok so lets say your playing a song and you get this kinda of thing 15b(18)
wat do u do ?
Thats a pretty healthy bend there!

Anyhow, what ^ he said.

Fret at 18 and listen to the sound. Then fret 15 and bend it while strumming till it makes the same sound that came from fretting 18. Once you have it, then you just bend to that point without all the 'test' strumming.