For me this supports my own experience a lot better than most of the other guidelines I've tried. I'm 5"11" and can't remember my inseam (shoe size 10.5 - 11), but every time I measure inseam and use the latest guide I'm looking at, I come up with a frame size of right around 55 cm - which is what I'm riding. The problem is that I always feel "cramped" no matter how high I set my seatpost (max. now).

When I used your formula I come up with a 58 cm c-t size and intuitively I was planning to just toss the 55 cm I keep coming up with and go to a 57 on my next bike. I think you're onto something. BTW, I have over 23,000 miles on two nearly identical frames that are 55 cm. and have always felt like they were a bit too small....its not like I'm on a new unfamiliar bike, or inexperienced.