Gday Don
The % rating of yr resistors doesnt come into it, unless yr resistors are going to change value during operation.
( You calibrate the result based on the resistors you have fitted, you dont even need to know their real values )
High resolution ADC will give you 1023 steps between Vdd and Vss. If you have Vdd at 5V, then each ADC step represents 4.9mV ( within the Pic ).
Assuming you have used a 3:1 voltage divider to bring yr voltage down, this equates to 15mV at the supply level. Fairly close to what you want.
When you calibrate yr chip you need to accurately measure yr PIC supply rail, and also the measured voltage.
Then its simply
Vact = Vpicrail*ADC/1023 * ( 4 * Calibfact )

Calibfact can be made to simulate a float between 0 and 1 using the ** operator, as such 4 * Calibfact should be able to get you near to yr approx 3 times stepup.
Set up a value and test the result from the pic against yr actual.
Adjust the Calibfact based on yr first measure using

newcalibfact = OldCalibfact * ( Actual Volts / Pic calculated Volts )

Should be apples after one iteration.
Andrew