Is there some way I can calculate for the EMI that I will be expecting?
No.
But what does the varistor do to suppress the EMI?
It shorts out voltage spikes. These could get onto the line by inductive pickup, it is all part of EMI.
Imagin the cable runing alongside the electricity supply for a lift ( elevator ), when the lift starts there is a surge of electricity, that generates a magnetic field round the wire, that induces a voltage into any wire running along side it. It is like a transformer.