If you have a '65 Reissue Twin Reverb, you have a bias adjust pot, but no test points, so you would need bias probes to measure the Cathode current for each tube. You can measure the Plate voltage at the output transformer primary, or get a bias probe that also measures the Plate voltage as well as Cathode current.
This video, although for a Hot Rod Deluxe, will give you a good idea of the procedure involved.http://www.eurotubes.com/eurotubes-Hotrod-Deluxe-How-To-Bias-Video-6L6-6V6-millivolts-mA.htm
The basic idea of setting the idle bias wattage, is to measure Plate voltage and Plate current and adjust the pot to get the current that will give you the desired idle wattage. Cathode current is close enough to Plate current, and is the measurement that most probes use, so just substitute Cathode current for Plate current in the calculation. Typically, you want to aim for an idle wattage dissipation of between 50% and 70% of each output tube's max Plate wattage dissipation. For example, a 30 watt 6L6 tube, you would want between 15 watts and 21 watts at idle. Plate volts times Plate current equals Plate wattage dissipated at idle. If you have 440 VDC on the Plate, and you want 15 watts at idle (50%), you would need to adjust the pot to get approx 35 mA Cathode current measured with the bias probe. 15 watts divided by 440 Plate volts equals .034 amp of current (34 mA). After setting the current, recheck the voltage and adjust as needed as the Plate voltage will change slightly with adjustment. After playing the amp for awhile, recheck setting for any drift that might occur.
I use the "Quadstage BiasPro MQ10" that allows measuring all four output tubes' Cathode current at the same time without a separate volt meter. This probe does not measure Plate voltage, you can do that with a volt meter. Other forum members will probably have recommendations of their preferred bias probes.http://www.asharpfretworks.com/5543.html