You ideally need to know the inductance of the load before choosing RC component values - the idea is that the RLC circuit formed when the contacts open absorbs the stored energy without going too high in voltage, and doesn't conduct significant current at mains frequency (the load is supposed to be off...). The voltage will still go quite high, but much less rapidly, allowing the contacts to separate before an arc can establish itself.
The resistance of the load is in series with any extra R you provide so that you take the sum of those resistances into the calculations.