Are you asking about setting the bias for a 50 watt plexi, 2 x EL34, one of Ceriatones kits?   
Have you built it yet?
Does Nik/do you use a 1 ohm resistor from cathode to ground?
Assuming the amp is built correctly, the bias pot is where it should be, and you can safely put your hands into 450 volts...
Check the plate current from ground to the 1 ohm resistor on Pin-8.   Lets assume a reading of 35ma (will be 35mv on your DC meter).   Now check the plate voltage on Pin-3.  From ground to Pin-3, Volts DC.  Assume a reading of 400 VDC.   Multiply the two numbers = 14 watts
I believe the EL34 is a 25 watt tube.   Assuming a safe dissipation of 70%, 17 watts would be your "maximum" desired output.   With the previous assumptions, you are at a very safe 14 watts.   You can raise the current, but at this point you should start listening with your ears and fingers to decide what sounds/plays best for you.   We will assume Nirvana lies somewhere between the 14-17 watt mark.   I imagine there is some cathode current to consider, but at this point you may just want to ignore it as a built in safety factor.
I have given you a guide, not a Bible.  This is the INTERNET.   You have no idea who I am, or what my qualifications are.   Keep that in mind.   Everything I have written could be erroneous.   Be smart, check another source.
Good Luck