low osc frequency
I would recommend using the built-in RC oscillator rather than building your own if you're worried about the timing accuracy. The internal RC is factory calibrated and probably far more accurate than your own RC will be, unless you use very high grade components. Generall +/-1% or less is posssible, and even better over the normal temperature range.
Most PICs with internal oscillators are calibrated in the factory, so you don't have to worry about it.
If you want to see the speed, make a program to output a pulse on a pin at high speed, and count the number of cycles that it should take. (including the loop) Have a look on a digital oscilloscope that can measure the period and you'll be able to see if its the right speed, and if it drifts at all. Heat up the circuit with hot air, or cool with cold spray. Compare your own RC circuit with the internal PIC one. I guarantee yours will not be as good.
In terms of the formula for the RC timing... check out the datasheet for the PIC... it says how to figure out the values.