All I can do is submit a basic approach.
The gain should be sufficient to bring operation back to a desired norm, yet not overcorrect. Overcorrecting leads to overshoots, and unwanted oscillations.
For instance, start with normal operation. Then an added load causes the sense wire to drop 1/2 V. You find you must add 2V to the control wire, to restore the system to normal. Therefore it looks as though gain needs to be 4.
Or, suppose a change in load makes the sense wire rise 1/2 V. You find you must subtract 2V at the control wire, to restore normalcy. Again gain is 4.
Notice it does not matter what are the absolute volt levels. Only the change in voltage is pertinent.