Yeah don't worry about mastery breakpoints. But if you want to calculate one you have to take 100 and subtract your base dodge PLUS 2 for dungeons OR 3 for raids (to account for the level difference of the bosses) to get value X. So if you want to raid and you have 14% base dodge, 100 - 14 - 3 = 83. Now X is 83. Divide X by the number of hits you'd like to take before you reach 100% dodge. 83 /3 = 27.66% mastery breakpoint. 83 /4 = 20.75% mastery breakpoint.
at the end of the day worrying about such trivial breakpoints is just super unimportant compared to choosing the right stats.