Yes, it can be confusing. Not because of the math but because you get different information from everyone. The issue is that tubes in general are "all over the place" and the specs for modern tubes cannot always be trusted. There....more useless information. 6550 tubes specs historically range from 35watts (SED6550C 1998 and JJ) to old GE specs at 42 watts. FYI, running the tubes at lower idle current, as I often do to extend tube life, is in no way harmful to the amp or the tubes. This hopefully makes things more reliable by causing less stress on everything. The "hotter" your tubes idle, the more stress you put on the tubes themselves, shortening their life, and also the power transformer etc. Notice most of the manufacturers tube specs rate their tubes at a lower screen grid voltage than typical guitar amps so that power rating is not exactly what the spec says. As an amp manufacturer, tube reliability is always an issue. We like to be on the conservative side in hopes of reducing tube failures. There are tradeoffs in everything. You will find the hotter you run your tubes, the more lively your amp may feel to the point where you really crank the bias current up and your amp really comes alive, while you burn everything up. While this may be true, there is a point where you will be shortening your tube life. So, from my experience, the settings we suggest are clearly on the low end with the goal of avoiding these issues. I have adjusted and listened to the amps with a range of settings and found the suggestions were a good compromise between tone and reliability. Your are welcome to try other settings though I do suggest keeping on the lower end of things if possible. Heat is the worst enemy of any electronics and the hotter you idle your tubes, the more heat is produced.
