Passively Increasing The Mana Pool ?

Hi again.

I need a passive skill that increases hero's mana pool.

example

level 1: +200 to maximum mana
level 2: +300 to maximum mana
level 3: +400 to maximum mana.

I did it with item ability "Item Mana Bonus"(converted to hero abilty).
It works but it can be leveled.Even i set 200 300 400 to level1,level2,level3 datas.It only gives +200 mana at level 1 , even if I level it it still gives the same bonus :S.

So I need your helps at this point , thanks from now on.

And you will notice max mana decucted according to its level when the ability is removed, even it didn't offer bonus properly. You can abuse this, or use a library to manually modify maximum mana.