Since I have two Rheostats and never use all four channels of either of them, I applied a ghetto mod to one. What I had available were a few 10 ohm 1/4W and 270 ohm 1/2W resistors.
Let's agree to number the 4 channels 1 to 4, left to right. The power lead is the center of the three that connect to the fan cables. On the rear of the connectors, I soldered a 10 ohm resistor from ch4 to ch3, and a 270 ohm resistor from ch1 to ch3. I then cut the Rheostat circuit board connection to fan connector ch3's center (which now has two resistors on it, and no connection to the mother board).
I discovered that I had a two-volt drop across the 10 ohm resistor, so a fan plugged into the newly-modded ch3, using the ch4 control, would only go up to ~11v instead of over 13V, using a 13.8V regulated Radio Shack power supply as the Rheostat power source. So I soldered two more 10 ohm resistors in parallel with the first, result 3.3 ohms and a .6-.7V drop, so I could now adjust over 12V, my objective.
With the 270 ohms connected to ch1 against the 3.3 ohms connected to ch4, ch1 becomes a ~100-1 vernier adjustment.
One final mod: the Rheostat outputs appear to be simple emitter followers, which will pull up but not pull down. So I added another 270 ohms (ya use what ya got!) from ch1 to gnd. That improved the range of the vernier.
So I now adjust ch4 to a stable voltage just under the desired final voltage on ch3's connector, and then turn up ch1 for the desired final voltage. This works so well that the only annoyance now is that the 3.5 digit dvm doesn't have instantaneous tracking!
