Far-seeker:
MOSFETs are primarily voltage based devices, so it's the voltage applied to the gate that determines how open the channel is between the source and drain. While MOSFETs do have a minimum current through the gate necessary to function, basically it's a very small threshold value. For most MOSFETs it should be well below the limits an Arduino output can supply. Now if you were using BJTs then the current of the controling signal would be a much more important consideration...
Gotcha! So its a voltage based device. Well, thats definitely helpful, but it brings me back to my original question: how do you calculate the voltage required to open the drain completely in a MOSFET based on Rds(On)? Is it just Ohms law?
What I'm finding is that, in the power range I'm looking for (25 amps), not all MOSFETs will have a completely open drain at the voltage and amperage levels the Arduino's pins can produce. Nearly all of them seem to have a threshold around logic levels, but I want full voltage potential at logic level.
Far-seeker:
The diode needs to be in parallel with the motor. That doesn't necessarily mean directly soldered between the fan motor's connection tabs, but if you put it else were you'll have to run the wiring appropriate to keep electically in parallel with the motor.
Fantastic! That answers my question, thank you!