Why not use a Pot? You could set a reference voltage quite easily, and you should already have a rather stable power supply for your PIC Wind the POT's to the required resistance and check them with a multimeter and your set. If for some reason your voltage regulator decides to conduct harder/less than the voltage Vref (the reference for your ADC conversions) on your PIC is also going to be affected proportionally. Eg; Vout from reg rises too 5.2 Volts, therefore the output of the potentiometers will rise by 0.2V but so too will what ever Pin (probably Vcc) you have as a reference for the ADC conversions, so your PIC will still read 2.26 or 3.673 volts (well the analogue equivalent), as its a digital representation of Vref