What level?
At level 280 (this is universal) (1 crit rate = 1 DEX) (Archers has a bonus of Level/5)
-30% is 200 crit rate
-40% is 267 crit rate
This is assuming no crit resistance, so it will be higher if you factor that in.
Crit formula is (Your crit rate - Monster crit resist) * 42/(Your level).