Voltage = (VDD*ADCreading)/(VoltageDividerRatio*1024) where:
1. VoltageDividerRatio = 5/12 = R2/(R1+R2) or whatever your voltage divider resistors are. Note that the source impedance looking back at the voltage divider from the ADC input should be ≤ 10KΩ, and that the AC source impedance should be lowered by bypassing the tap on the voltage divider to VSS with ~1uF capacitor.
2. VDD is the actual PIC supply voltage, to three significant figures.
3. 0≤ADCreading≤1023