Prot Warrior is the one tank class I cannot learn. Fuck Ignore Pain. I just can't do it.
I figured it out a while back, it's basically a limited bubble. Shield Block always takes priority, I remember that I'd take heavy repercussions so that I could time Shield Slam and get more up time on Shield Block.
Then using the Vengeance talent to synergize Revenge with Ignore Pain. Basically I'd only ever use ignore pain after a revenge, to reduce the cost of ip, then revenge again when it's about to run out, as ip also reduces the rage cost of revenge. But only ever use revenge after a dodge or parry, of which there tended to be many. So a dodge or parry early on would set me up all fight, so long as there was no tank changes needed.
There was actually an add-on that people used to use to figure out the optimal way of deploying revenge/ignore pain. It'd basically tell you when the ideal time to use each ability was, based on the talent's changes to both abilities.
After a while you just got a sense of when to do what. It's definitely more complicated than it was back when I was a raider though. Back then it was just shield block and the occasional spell reflect.