if using the appropriate reload and preload as calculated is not keeping good time then it means you xtal frequency is slightly off (as pedja089 suggested 1 to 5 % error is to be expected) , you might try trimming the capacitors on the crystal (might take a bit of experimenting if you haven't got a frequency counter) , changing the preload + or minus 1 will probably be too coarse an adjustment . you need to keep in mind that xtal ocillators aren't atomic clocks and that the osc frequency will be temperature dependant and will drift over time .
another possibility is adding or subtracting the "error" say once per day, this was used in a slicon chip magazine project (pic toc )many years ago and was suprisingly good at time keeping .