In theory, if you divide the change in rim width by pi (~3.1416), you'll get the change in mounted tire width. In other words, a 3 mm increase in rim width will give you about a 1 mm increase in mounted tire width. The real math is considerably more complicated, but this is a decent approximation in most cases. Since it sounds like you're already close to zero clearance, I'd be really cautious about changing rims.