You actually take the measurements for the snubber last, oddly enough.
I'm afraid snubber design is part engineering, part black magic. For your application, it's just a resistor in series with a cap across the motor drive - sounds simple, huh? What it's doing is killing a high frequency inductive spike that originates from a complicated interaction of things, reactances that include the lead wires and distributed capacitance in the windings, switch times from the drivers and diodes and motor commutator.
Simulators are useless for this, you have to get in there and measure it. You need a scope. You start off with a low drive voltage and crank it up until you see dangerous spikes. Your snubber RC time constant needs to be a small fraction of your minimum "on" time of the pulse width modulator or you are snubbing the PWM. The resistor is "burning off" the energy in the spike, the capacitor is storing it.
And just to make things more complicated, there are invisible spikes that are just as deadly. Current spikes. You can get a feel for them by measuring the voltage developed across a shunt resistor... but that small resistance affects the high current spike. A current probe is better - if you have one, and if it's not the moral equivalent of a snap-on ferrite.
So you damp it as close as you can to stay inside the SOA curves (which are missing from that lousy spec sheet!) without introducing too much capacitive load to the driver, which sends up the current spikes.
On a brighter note, you might not even need one. That MOV may be designed to deal with the voltage spikes. When you put the 2 drivers in parallel, the peak current spike it can handle is a thousand amps - just for 80 microseconds, but that's in the range of these current spikes. That's why they give you that number, it isn't for motor stall and start.
Inquire with the manufacturer about that "polarized" MOV, see if it was intended to go on the drive's output, or supply. Is there an SOA curve, or are we just supposed to guess? Are there numbers stamped in the plastic that aren't on the spec sheet? Right now I can't even see what pin goes to what!