The potentiometer doesnt' go on / at the shunt. It goes on the output side of the electronics that measure the shunt, on the line from those electronics that go to the MCU chip in the controller that runs everything. Exactly where this line is, you would need to locate by tracing out the PCB between the main MCU chip (usually the one with the most pins) and the area where all that circuitry is near the shunts. The potentiometer would be wired so that one of it's outer leads goes to ground. The middle lead goes to the MCU line. The other outer lead goes to teh line coming from the electronics measuring the shunt.
What the potentiometer does is simply let you adjust that signal downward (lower voltage) so the controller thinks there is less current than there really is. Typically you would adjust the potetiometer first so it is outputting the full shunt signal to the MCU, then begin adjusting it downward until the controller either does what you want it to, or smokes.
The potentiometer doesn't handle any current (probably microamps at most) so even a tiny little pot will work fine. Typically the size of your pinky nail, or smaller, usually one of the blue multiturn pots with the tiny brass screw adjust for fine tuning, easy to glue down to a surface on one of it's flat sides. 5kohm to 10kohm would be typical value used.
The posts by Fechter et.al. about that kind of shunt mod have more info on how to trace this out and connect it.
However, if you have modded the shunt itself, the potentiometer is not useful, because the shunt no longer does what it was meant to, and the reading the controller gets will already be lower than it should be (or nonexistent, depending on the results of the shunt mod. The resistance of the shunts is so low even before a modifcation that you can't measure it with a regular multimeter, the meter leads have more resistance than the shunts do, so that's what you'll see. The shunts are probably a thousandth of an ohm (1milliohm), or even less, and will be much less after soldering wire to them (basically just that of the wire and solder, so they aren't actually shunts anymore, just wires, and can't do the job of a shunt). Even removing the mod doesn't put the shunt completely back to where it was, becasue you can't remove all the solder from the surface, so they will still be some uncontrolled lower resistance value than they were originally.
It is still better than with the mod, but not the same as it was, and the effect is not completely predictable (it is measurable though).
You *can* measure the actual shunt value using a constant known current thru them, and then measure the voltage generated across the shunts. Then use ohm's law to calculate the shunt resistance: Volts / Amps = Ohms.