Yes...my method is a gross oversimplification. But for a setting that's very subjective, starting in the ballpark of 35mA seems to work and is safe. I usually do tweak up somewhat but not to where the plates are glowing.
I guess another thing that should be mentioned at this point is Groove Tubes. (not that I use them...but just so the conversation is complete) GTs are rated with a numbering scheme (1 - 10). Once you've had a set in your amp and had it biased, you can just order the same value tube. So, if your amp was biased with a set of 4s, you can be reasonably confident that you can slap in a set of 4s the next time. The point was to make it easier the musician to retube without a trip to the amp tech. Conceptually, I think it's a great idea. However, you are limited to their tubes which are just tubes from other manufacturers that have been rated, labelled, and repackaged (for more $$$)
However, I've found that I have found better sounding tubes through recommendations on the web and doing my simplified bias adjustment. Currently, I'm very happy with a set of Visseaux 6V6s in my '65 DR. They were recommended by Lord Valve
http://www.nebsnow.com/LordValve He's always had excellent suggestions for tubes.