So what you were looking for was not really to calibrate it, but to reduce the inherent error that occurs when using a close-enough (but not exact source) for the BAUD generator. This makes more sense (since the Oscillators are trimmed at the factory for 8MHz, before they leave the factory).

Note that 4MHz gives you 0.16% error on the 19,200 BAUD generator. On the other hand, 3.9936MHz is right on. So maybe this was the answer you were looking for through experimentation.