Reporting question for Bias Master™ - TAD, tube tester, with 4 probes

Bias Master™ - TAD, tube tester, with 4 probes

I read the bias calculations and understand how in your example to get 70% of I (bias voltage). In equation I = P/V. I see that value for P of 14W is listed for the 6V6GT right on your .pdf file that will also be included if I purchase the Bias Master. However, you stated you measured V, the plate voltage (420V) in your example. (as opposed to using measurement chart shows V to be 300 - 390, so it seems measurement prevails) How would I find that value out myself without any additional equipment ? Also say in 100W Marshall P is listed as 25W but for EL34's, but If my Marshal 100W amp is using 6550's can I use the P value for Ampeg SVT since that amp lists 6550A as that tube type ?