Designing an RC Snubber for TRIAC motor control
You really need an oscilloscope for that kind of thing since you're basically working blind without one. Snubber calculations need to take into account the motor inductances in the circuit so without proper instrumentation you can't really do anything. One method involves just do paper napking calculations then running very carefully at low power and observing the ringing frequency and using that to narrow things down. Then you fiddle with values until the problem goes away on the scope.
If you really are working blind, there's no reason to fuss over it so much since you can't verify anything anyways. I'd just pick a 1-5Ohm resistor and a cap as close to 100nF as possible and just live with it.