For the most part i like to to got roughly 110/120% mastery and then go for crit.
Gives me a good amount of mastery for those “Oh shit everyone is dying” moments in progression and then crit for the rest. As for how much crit, I just get enough for me to last an encounter 5/7 minutes long. Sitting at 20%ish atm, aiming for 25ish when mythic progression starts.
Ofc, this is just my opinion and i know alot of people just go after as much mastery as posible.
Hope it was an answer you could use.