I use a drafting circle template which can be purchased at any drafting or office supply store. I first paint the whole tire black. Next, I match the closest hole size to the rim size and then tape the tire to the template. Make sure the rim is centered in the template. I tape off the surrounding holes on the template for cleanup and to make sure no paint dust can find it's way to the tire. Then I make a couple of LIGHT passes with the airbrush until I am satisfied. As long as the airbrush passes are light, I have been very successful in creating nice rims - no matter what the size.
On another note...If you are using a mask, I would suggest that once the mask is in place, lightly brush some Future around the edge of the mask and let dry. This will prevent bleed under.