The gain is defined as the difference between the first and second photoelectron peak positions. At our nominal operating point of 6.5 V and 6.5 K, the gain is 12,400, with an rms of 15%. The rms of the normalised gain distribution varies from 9% at T=6.0 K, U=5.9 V, to 24% at T=7.0 K, U=7.1 V.