Len, when setting the bias, do what sounds good, in any given safe rating for your output tubes type.
that being said, you'll have to know the safe operating range of your output tubes
if your amp has a plate voltage of say..500 volts (just to make it easy mathwise)
and the tubes u are using are rated at 30 watts maximum dissipation and, going by the
general consensus that biasing tubes anywhere between 60-70% of maximum dissipation is optimum (tube life, tone, etc). so u would use the following equation
30watts / 500volts = .06
.06 (from above) x.06 (or 60%) = .0036
so 36mv on your meter
.06(from above) x .07(or 70%) = .0042
42mv on your meter
so, anywhere between 36-42 is considered optimum (between 60% and 70% of max tube dissipation) tonally, in some circles/general consensus
use what sounds good. anything below, just extends tube life, gives less volume and less output tube crunch (pending of course on tubes, volume, setup, etc). when im jamming low volume at home, i lower my bias. the power tubes are just power in that case, not really needed to add anything tonally for what i do there.
so, to rehash, for an amp that biases to a certain safe working range to the tubes being used, you need to know the plate voltage, and where to plug it into the above equation.
it's useful if your amp is 6L6 stock, and you switch to something else like kt66 or 5881(the sovteks i have, for all given info i can find are rated at 25 watts).
if you're using 25 watt tubes with a plate voltage of say....495 like i am in my 5150
then 25/400 = .0625 or .063
.063 x .60 = .0378 or 38mv
.063 x .70 = .0441 or 44mv
so between those 2 ranges gives me 60-70% maximum output tube dissipation.
im sure someone else will chime in here and correct all my wrongness (terms, math, etc), but this is what i've gathered on my own how to figure it out from a bunch of different sources.
plate voltage is key to know for your amp if you're in there a lot, or changing tube types a lot....but something to keep in mind, it also fluctuates with the fluctuating AC coming from the wall you're plugging into. and, most likely, once you get in the sweet spot that you like bias wise, you're not going to notice a difference tonally between a few mv in the settings. it's a compounding thing....not like something you'd notice drastically with each mv increment...hard to explain what i mean...you'd get more results chasing preamp tube dragons than you would fucking around with power tubes and bias settings, IMHO for an eggie renegade or tourmaster (like bruce told me damned near 3 years ago, but i had to find out on my own! LOL).
if you stick with one tube type, find a bias setting that works for you, and stick with it!