Yeah just chiming in to back this up. I basically use my common sense in this kind of situation. Let's say seraph is 3-4 seconds off being up and I have 5 HP. I just got an Avengers Shield proc and Crusader Strike is 5 seconds off cd with Judge a second or two behind. I'm definitely going to hit the SotR and spend the HP, because I can clearly see that I will get 5 HP within 2-3 seconds of Seraph going up but I'll definitely lose at least 1 HP by delaying it and for all I know I could get another AS proc in the next couple of seconds which would be similarly wasted by pussyfooting around waiting for Seraphim cd.
Obviously this all goes out the window if the boss isn't hitting you, it's a tank swap, the boss is casting something you can't interrupt or 100 other reasons why you don't need hte SotR buff up right this second but I think you can see the thinking process behind this.
TL;DR you should be able to work this out yourself based on what else is going on in the fight.
And if none of this makes sense and I'm completely wrong (wouldn't be the first time) everyone feel free to point it out
Nope your doing it right. Made perfect sense.