You need to know plate voltage and how the tube is biased to determine dissipation and how hard the tube is run. They won't tell you. So simply look at the watts per channel, and divide it by the number of tubes. 80 watts per channel out of two KT88's. That's 40 watts per KT88. The maximum dissipation of a KT88 is 42 watts. So that's toastier than you might want.
Kevin, in your example I quoted above the likelihood is that the tube is dissipating much less than the rating! In this case it might only be dissipating 20 watts at full power.
The dissipation is important- it gives you some idea of how much power you can dissipate in the tube. But if the tube is producing 40 watts of output power, there is a larger amount of power that is needed to cause it to do that. Typically it might be about 60 watts or so.
A better approximation of how much power is being dissipated in each tube is to look at the total power draw of the amp at full power. Subtract about 7% for the power transformers themselves. Then subtract about 10 watts for the driver circuit. Then subtract the output power. The remaining power is divided by the number of power tubes and will be pretty close to the dissipation of each tube at full power. Some variance will exist on account of the filaments and class of operation.