Cool downs are situational, that comes with experience in tanking and knowing the mechanics of the encounter be it trash or boss. Your opener should be (blood for blood if you really want to but its extra damage on you too) Flight or Fight, Shield Lob, Circle of Scorn, Flash (depending on how confident you are you can flash twice) and then go into your rotations, remember to put in a shield swipe when it procs.

Enemies become immune after your 3rd flash or shield bash if the blind effect or stun actually works on them.