i dont really think there is a min crit % for arms, it just naturally comes from gear/spec. 33% crit is fine i think. when i made the switch from fury to arms i had about that much with axe spec until i got some ulduar gear.
just dont gem for it, let it come from gear. i usually pick up agi necks/rings to help out a little with crit too (most are bis anyways).