afaik tao adds 100% hp of the base hp+vit hp, (even with + vit effects of cards and armors or buffs whatever) if i can call it like that. i hope you know what i mean.
same with other +% hp gears.
imagine you have 10k hp without +% hp gears. you put on dl shoes. they add 50% of 10k, so you have 15k hp. now you put a tao on and it also gives 100% of the 10k you have without those gears. so you have 25k with both of them.
understood?

edit: skills which increase hp might be different. i know that some can add the +% hp to your real hp with sleips tao and so on