Choosing RC values to produce specific frequencies

I'd like to run a 12F615 at ~2 MHz instead of it's lowest internal clock of 4 MHz. Precision is not important, so I thought I'd use an external RC network. However I can find no information on how to choose values. (Well, there is a recommendation to be not larger than "X" or smaller than "y", but nothing that shows a formula to calculate frequency from known RC values, or vice versa.)
Randomly stuffing values in my breadboard gets old pretty fast - does anyone know the magic formula? I'm sure it's in some datasheet - just not the one for this part.

Hi, Charlie

The result is variable from a chip to another ... so no formula given :rolleyes:

but try F = 1 / ( 2.4 x R x C ) ...

so, 10k and 22pF should be close to 2 Mhz ...

Alain

Thanks, Alain!

This was very helpful.
Can you tell me where the number comes from? I tried to derive it several ways, but no luck.