However, splitting hairs aside, the difference between favoring crit and mastery is so minute overall. Going GCD cap/high mastery, vs DP3 and a balance of crit mastery is within the statistical margin of error... so as long as you have the 2pc bonus, really its only having at least the DP3 haste breakpoint that matters, after that, you can pretty much have stats however you like.
Otherwise, a simple example for the OP (assuming 1% crit = 1% mastery in overall benefit):
If I have 20% worth of stats to distribute, I could do:
20% crit = 1.2 X Damage
20% Mastery = 1.2 X Damage
10% Crit, 10% Mastery = 1.1 x 1.1 x Damage = 1.21 X Damage
Because crit and mastery have a multiplicative benefit, splitting between the two gives more of a benefit than dumping the pool into one side.