If you choose a Pic that has the capability to use external Vref you can get about twice the resolution(above 15V). Put a 50/50 voltagedivider(2.5V) on Vref-, feed it with Vdd. Connect Vref+ to Vdd. The AD will now give you a zero reading for everything below 15V. 30V will still produce maxreading. Your calculation would look like .......

ScaledResult = (AccumulatedADvals ** 15000) + 15000

Unfortunatley the 12F675 doesn't have this capability.