Todd,
Keep in mind I'm a bass player so I don't know nothin' about strings smaller than an .045... I'm just trying to figure out the physics of your problem.
My feeling about the nut is that it should be like a zero fret (this might just be my preference, others may disagree). In other words, I think the open G-string should clear the first fret by about the same distance as playing a G# clears the second fret. This way you are not deflecting the string any further than normal and pulling it sharp. Make sense?
However, since your changing to a .021 fixed the problem my idea probably doesn't apply here. UNLESS the unwound .018's additional tension is upsetting your entire guitar's action. I guess that's possible but you would likely notice it higher up the fingerboard.
All the guitar players I know are VERY specific about the string gauges they use so that's a personal thing. Figure out which combination you like the best, sounds the most even or full or whatever you're looking for, and then make the guitar cooperate by adjusting the setup accordingly.
Good luck with it,
Jimmy J