So my road bike with a 105 triple front DR, and 105 rear DR/Cog-set only has 20 usable? That's 30-10=20? That number seems low.

Let's start with the middle chainring. On that ring, I have full range of use of the rear cog set. So that's ten.

Next, look at the large chainring. On that ring I have the use of the smallest cog, all the way through the 3rd from the largest. After that, things start getting noisy. So we'll call that another eight.

And finally consider the small chainring. With the small one, I am able to use the largest cog, all the way to the 2nd from the smallest. That's nine.

Total: 10+8+9=27. Ok, the 2nd from the smallest on the small chainring may be impractical, even though it works ok on my bike. So we'll call it 26. Your formula was only inaccurate by 6, which is off by 20%. I'd say it's flawed.