Dodge = 25%
Parry = 20%
Block = 15%
Miss = 10% or something, obviously all these depend on your relative avoidance values, this is just an example
Dodge then takes up 1-25 on the table, parry takes 26-45, block takes 46-60, miss takes 61-70, getting flat out hit takes up 71-100.
etc. etc., the only way to get knocked off the table is to have more than 100%
(This is more for all those people wondering wtf we were talking about) If you were to do a priority like (check dodge, then check parry, then check block, stopping whenever you hit a check) then each attack would require tons of calculations causing blizzards servers to crap themselves. Instead, they just do 1 calculation with a table and avoid all the annoying statistics crap involved.
As for not using blood draining because it's not game breaking, that's like saying you might as well gem socket bonuses for dps since 10 stam isn't going to matter for survivability in the grand scheme of things. Blood draining has saved my life as verifiable by WoL reports. Why you would choose a dps increase for a survivability increase, no matter how minor, baffles me. A tank's #1 job is to survive, let the dps do the dps. Gear/gem/enchant for survivability, no matter how marginal.
Edit: Also what MellvarTank said.