Runes

Might be wrong board, but in order to expand my runes and rune pages I'd like to ask a hopefully simple question.

9 flat HP seals VS 9 %hp seals.

Now, flat would be 72 hp (if I'm not mistaken)

%health would increase by 4.5% in total.

So, does % health work off of all health built, or just base stats. In which case would % health be better than flat (if ever)?