There really isn't a % on what to enter. Just if you want to play it safe without having to rely on Cors permanently. Smns can damage pretty well when the Rate is 26% or higher.
I'm pretty sure he meant what's the lowest the NMs health % can be before you are ineligible for rewards at the end of the fight.
A friend of mine joined a Hurkan reive while it was at 48% and got nothing but the KI at the end of the fight.