Personally I would change Constitution so that it reduces damage taken instead of increasing health. (Which is actually what I do in my RPG.) As someone who usually picks tanky characters, I'm not too fond of stacking pure health. I know that a lot of old-school RPGs have that kind of system where constitution/vitality/stamina increases health and intend that as THE primary tank stat, but personally I never found the intended way to build a tank to be the best one.
Having a lot of health just delays your character's death, and it makes him or her harder to heal. What do you prefer? Healing a character that has 100 health and 50% damage mitigation, or a character that has 200 health and 0% damage mitigation? Mathematically, they take the same amount of punishment, but the first one needs to be healed half as often as the pure-health tank.
As it is, if I was playing your game, I would probably get just enough health to survive magic attacks, and then put all my attributes into evasion so my tank can get some crazy 90% evade, so that the only thing that can hurt me is magic damage.