If you are going to armor swap, my best advice is to only use it when you are sure you will be able to have trinkets up, an example would be on the pull of any fight, but you can also use it after long periods of not dpsing where there is a good chance of trinkets proccing, I armor swap on the second transition of lei shen so that when I pop my second pot, lust and likely both of my trinkets are up, I can take full advantage of mage armor a second time