Man....still a countless # of people who don't understand tanking mechanics.
I you have 50% dodge & 50% block (no parry), you do not have 100% avoidance. You have 2 independent events of being able to avoid an attack. This amounts to 75% avoidance (which is rather intuitive in this case). Take a probability class, read a probability book, or go to wikipedia, inform yourself, and use the formulas as you like. I mean come on, that's like telling me if I flip two coins (50% chance of getting a heads each) that I'll be guaranteed to get 1 head if I flip both (50+50). Apparently it's impossible for both heads to land on tails .. .. /facepalm.