First off, the math and measuring is only important to a certain extent (staying within reasonable limits) -- the most important part is how it sounds. I would suggest starting at about 70% of the max plate dissipation of the prescribed tube.

different plate dissipation ratings:

Examples:

EL34 max plate dissipation 25w

voltage=400v (this is the plate voltage -- this should be what your meter is measuring for voltage)

.7*(25/400)*1000=44ma
(% of max) x (max wattage/plate v) x 1000 =bias current

6V6 max plate dissipation 14w

voltage=400v

.7*(14/400)*1000=24.5ma
(% of max) x (max wattage/plate v) x 1000 =bias current

I don't know what your plate voltage is so you'll have to do the actual math. I would use this only as a starting point and you should really go with what sounds the best and not be consumed with the numbers because that's what truly matters. So, experiment......