[ARC5] Question about running the SCR274 transmitters on lower voltage.

[J Mcvey via ARC5]

I have a DY401 which puts out 440V at 180 ma which I want  to use with the transmitters for lack of anything better. Has anyone on the list tried this? 

If so, what is the "magic number" for the screen resistor at this voltage? The specs say that the max screen voltage is 350V, so maybe I can set it at around 300 to be safe? Maybe I can get 50W out?