Incidentally, my number (0.777) wasn't pulled out of a hat as I've already explained. It is, however, based on different assumptions about mirror reflectivity and coatings effectiveness than those that Jon used in his example. It is also seemingly closer to "reality", perhaps by accident. That is, it may be based on pessimistic estimates of AR coating efficacy and mirror coating reflectivity, but coincidentally such pessimism may be warranted by other factors not accounted for by the model (mirror coating deterioration, internal vignetting in catadioptrics, spider vanes and light lost from the focused image due to scatter from things in the light path, etc.
Actually, when I computed your example of the C-6, I used the numbers for the reflectivities that you provided for the Starbright XLT coatings, you chose to use the reflectivity for the standard coatings...
But in any event, I will say this: There is a great variation in the performance of telescopes. A well thought out reflector can be an amazing performer and very efficient, a well thought out refractor can be an amazing performer. Poorer quality examples of both exist.
But if one wants a rule of thumb, one digit is sufficient, the rule of .7, the rule of .8, the rule of .9, whatever. .777, totally unrealistic. I suggest a better set of rule would be a fraction... 1/2, 2/3, 3/4, 4/5, 5/6, 6/7...7/8... 1
Take your pick, it'll apply to some scope.