My templar finishing move description is as follows : " does up to 480% more damage when the target is below 50%". But i m sure that the bonus is not at 480% when the mob is at 50%. It seems to me that the bonus evolve from 0% at 50% to 480% at 1% (it is a guess, may be it is a linear scale may be not.
Using it is a bit annoying because it is channeled.
I usually try to use it while maintaining my buff and then putting the spell that record damage and deliver them in between my finisher.
But i don' t know when it starts to really pay off, probably not at 50%, may be around 30 ....
If anyone did testing i will be happy to get some data or formulaes.