Hi,
Point B as Vref- was my initial thought as well. Then I noticed you're driving the thing with 24V and 24V on the PICs pin is not a good idea (the PICs Vss is still connected to ground, right). The internal clamp diodes will conduct and you'll likely destroy the PIC. You could limit the current going into the PIC with series resistor to prevent it from being destroyd but you'd still not get any reliable results.

Why do you think the PWM wouldn't work with the resistor "below" the MOSFET? As long as the resistance is low enough so that the MOSFETs Vgs is respected I can't see any problem with it.

It says 20A and 1ohm on the resistor, that's a 20V voltage drop across the resistor which is A LOT. If you're really driving 20A thru it somethiong like 0.05ohm would yield a voltage of 1V.

/Henrik.