[Amps] 813 cathode zener bias

[Steve Flood]

I am considering building the adjustable zener bias circuit as used in the GM3SEK triode board to try in my 3x813 grounded grid amp project.
The GM3SEK documentation says the cicuit is good for 3 to 27v of bias, with provisions for changing it to 27-45v.
Without any published tube charts for 813 g-g operation,  how much cathode bias voltage range do I need if I wish to experiment with AB, B, and C operation?

Thanks,
Steve KK7UV