No, it doesn't work that way, Arredondo. What happens if you use your equation and a value of 50 ranks of Dagger Mastery?
50 ranks * 0.2 per rank = 100% chance of a double strike. Meaning you attack twice as fast 100% of the time.
Using your equation that would be 1.33 - (1.33 * 1) = 0
If you attack with double attacks 100% of the time, your effective dagger speed would not be 0 (as your equation would predict), it would be half of 1.33 (0.665).
Let's try 50 ranks with my equation:
1.33 / ( 1 + 1) = 0.665
See the difference? My equation for effective speed is correct.