A snubber would help by giving the flyback current a path to flow during the period when the relay is switching over when the contacts are open which would reduce arcing.
But no RC values can be given because you need to know the inductance of the motor and a few other things. It's much easier to go trial and error. However, I will say this. I don't think you need a resistor. THe resistor is there to limit the power dissipation, but your event only occurs at a very low frequency, not kHz like PWM so the energy dissipation is much less. So you could probably use a much larger capacitor for better damping (which also increases power dissipation) but your low frequency offsets this. A resistor might still be needed, but those are easy enough to adjust.
Or you could use a bidirectional TVS diode that is rated a little bit higher than 12V in parallel with the motor. It will breakdown and allow the flyback current to flow between motor terminals through the diode rather than by arcing across the contacts. Make sure it's the proper power rating to survive. A bit hard because of the way TVS diodes are rated, but your application is very low frequency so that should help.