[Fantomen]

Possibly the 30% is based on the lifespan before fire magic penalty is applied, making it seem like closer to 50%?

Let's say fire magic reduces by 30% you'd have for example maxage 100 rduced to 70. If a nature pick boosted this based on 100 you'd go back from 70 to 100 which is closer to a 50% increase from 70.

Death magic does not affect lifespan or old age itself, but reduces the chance to get afflictions from old age. So it should not be a factor here.