VGPU
SC1175 - 10k pot between pin 18 (-IN1) and pin 20 (GND)
Default: 1.5v - 1.65v
Suggested maximum:
- Air-cooled: 1.75v
- Water-cooled: 1.85v
- Extreme-cooled: 1.90v

VDD
ISL6522 - 20k pot between pin 5 (FB) and pin 7 (GND)
Default: 2.7v - 2.9v
Suggested maximum:
- Without ramsinks: 3.2v
- With ramsinks: 3.6v

VREF
ISL6522 - 50k pot between pin 5 (FB) and pin 7 (GND)
Default: 1.4v
Set this to 1/2 your VDDQ. More than 1.5v VREF will probably kill your card.

VDDQ
ISL6522 under red/silver plate - 50k pot between pin 5 (FB) and pin 7 (GND)
Default: 2.7v
Suggested maximum: 2.9v

Use the middle and either the left or the right pin on the pot. There is no polarity you have to watch. I strongly suggest removing the unused pin so you can't accidentially put the potentiometer there flipped and kill your card with too high voltage.

Start with the pot set to the highest resistance (turned all the way from the outer pin you are using) and slowly work your way down while measuring voltage- you can safely change resistance while your system is running.

To measure voltages attach the negative (black) probe to your case or another grounded part and carefully put the positive (red) probe on the measurement point. Be careful not to create a short with another of the 'posts' near the measurement point.
A better way is to attach an SMD grabber.

When removing the red cooling plate remove the white pins first (gently push them through the holes). After that heat the plate with a heatgun or similar to make sure the heatsink glue ATI uses is soft. If you don't do this you risk to rip an IC off the PCB when removing the cooling plate.