Thanks ja22y!
Done the mod, took the voltage, did the maths.
50 & 100 watt modes there is little difference, I'm reading 419 volts and biased to 45mA which has got me round the 65% or 19.5w mark.
It turnes out I'm very cold with the valve rectifer on, 376 volts and 24mA! I should be getting...