Tank item idea

So, I have been playing tank and found it annoying how theres no way to counter % hp true damage, and while I know it's important to keep tanks from being overpowered, I think I found a way to reduce that annoyance while not making it redundant in form of an item. So these are the stats (numbers can always be changed): 400 hp 60 armor Unique Passive: % Max hp damage is limited to 30 per 1% Max hp damage. (So if you have over 3000 hp, 1% Max hp true damage only does 30 damage, 2% does 60 damage,3% does 90 damage and so forth) I would say the item is only useful lategame since the Effect only becomes noticeable at over 4k hp, but with 5-6k hp for example, these certain abilities hurt a lot less. It wouldn't counter just certain champions, just Scroll down this site and you can see all the Champs with % of targets Max hp damage: http://leagueoflegends.wikia.com/wiki/Health