Interesting, I've only ever seen it goes off when both are less than half...I'll have to look more carefully. I think the issue of you not seeing it trigger when a lot of big hits are flying around is there is less opportunity for it to proc since your glad receives few hits. Little hits allow more time and rounds and increases the probability your glad will use the skill simply because he has more opportunities to use it.Not true at all. My war glads have triggered NSD plenty of times with 3/4 endurance or more many times. I see it rarely triggers when consistent big hits are being landed. The majority of the time it seems to work when medium to light hits are being taken or light bleeding is occurring. When big hits are being taken or the bleed rate is high it seems it doesn't trigger much at all.
It's really just odd how it is so difficult to gauge.