When I was hero and I was wearing hero cloak and weap I was near 34k hp and I never seen activated sense of royality. But now I wear bow+heavy and Iam on 13k hp (in Kartia without sonata) and almost every hit from Kalios turn me on Sense of royality.
Maybe its depens on % of you hp, lets say if you got hit for 20-30% it activated. So less HP = you need less dmg to receive to activated it. I didnt test it, its just my thought.
Another though is that you need get some amount damage in time. Like if you got 5k dmg in 2,3 sec its activated.
Maybe it would enough just take armor off and let healer summon hit you :)