Calculation of varistor parameters for AC line protection
The V275LA20AP varistor is specified for up to 275 volts RMS and the data sheets ties this in nicely when multiplying 275 volts by approximately \$\sqrt2\$ to get 287 volts. This is the minimum voltage that would cause 1 mA to flow into the device. But, you could get a device with the same marking that only draws 1 mA at a voltage of 473 volts (22% higher). 

This makes me conclude that the clamping voltage of 710 volts (50 amp surge) might in some cases be 22% higher at maybe 868 volts. You need to consider this if your protection is to be effective.

But, is a 50 amp surge good enough? In terms of standard lightning protection specifications, EN 61000-4-5 testing can inject hundreds if not thousands of amps. This makes me ask you if you think you are designing using the right component.