Hi,
I don't follow you on the math....
If you reload TMR0 with 12 each interrupt you'll get (256-12) * 256 = 62464uS between each interrupt.

What kind of accuracy do you need? With the prescaler set to 1:256 and a TMR0 reload value of 178 you'll get (256-178)*256 = 19968uS between each interrupt plus a little for the time it takes to reload.

Another way would be to use an external xtal for TMR0, with a standard 32.768 watch xtal and the prescaler set to 1:128, you'll get one interrupt each second.

MPLAB have a simulator where you can run the code, it has a stopwatch function that can be used to "measure" how long a certain piece of code takes. I've never used it myself.

/Henrik Olsson.