By setting the damage percent to 49 for not normal difficulty, it works for both hard and insane difficulty levels.Just a hint - there are scripts in the vanilla game that may provide you with the similar issue.
E.g. ActionOverride("hexxat",ApplyDamagePercent(Myself,70,FIRE)) for Hexxat
or ApplyDamagePercent(Myself,60,FIRE) for the creature in the beholder quest temple
The difficulty setting is supposed to only effect enemy damage and not all damage. I think perhaps the way those lines are written may be considered self damage and so could be fine. I have never had any issues dying unexpectedly during game scripted events in the vanilla game. But I haven't played vanilla in a very very long time so I don't remember if I played on increased difficulty.