Bends are a strange thing. Unlike Piano, Clarinet, saxophone or most other instruments, guitars can often hit notes in between the actual "named" ones. If our ears were adapt enough, we could make the guitar have an infinite number of notes.
I have a couple of solutions for you...
1) a 1/4 bend shud be half the distance from one fret up. You can train your ear to find out half the distance between notes. try playing a note on the 1st fret, then on the third. now sing what u think wud be in the middl. Play the note on the 2nd fret to see if u r rite. Once you get good at this, try to do it with larger (1st fret and fifth fret) and smaller scales (1st and second). Now try to do it with the note u r bending, and 1 fret above it. This may not work, its just a guess.
2) Try to see the distance that you bend for a 1/2 bend, then only bend half that distance, this will work if your guitar has really good intonation, and you are precise on your bends.
Good luck man!
There are only two important things in life - There's music and theres girls, not necessarily in that order....
The Ace's Guitar Tricks