Do you have a Bias Probe?
If yes, then the recommended current is 22mA per tube as noted in the schematics.
If you do not have a Bias Probe then it is getting a bit more complicated as you need to utilize Shunt Method by measuring Voltage dropped over the Putput Transformer Winding which is a method normally only for the experienced people as there is high voltage in the game. If you are confident of doing that then it is just simply measuring resistance of each Winding half (between Pin 3 and CT, for an 18W OT something around 300 Ohms), applying Ohms Law and calculate U= R* I with I = 0.022A and adjust so you achieve the calculated Voltage,
Regards
blues
