Actually you don't need to assume 0v. It can be 1v for all the divider Resistors care... choose the values for R2/R3 that will give you 3.3v/3.0v when that pin is LOW, if you assume 0.6v then you will be pretty close with a paper exercise. If it's a one-off build (or a product where a high degree of accuracy is required), the using Hi-Stab multi-turn pots and manually calibrating as I suggested is the way to go...

Either way, it's just another suggestion thrown into the melting pot of ideas... another could be using an LM317 and using the PIC to switch-in different Resistor values for different voltages (depending on the current draw, that option could also power your Sensor as well as Vref+)... one could carry on throwing ideas around all night long...