If you put let's say three Superior Magic runes on your armor your health will drop by 225HP, since 3 Sup runes at -75 HP each subtract 225 HP from your base HP (480)resulting in 255HP
Add a Sup Vigor to it and your HP will increase to 305 again.
The situation will decide what you need to do but a common way of doing things is to get a headpiece for every magictype and stick the according Sup rune on it.
That way you always have one maxed attribute without loosing too much health.
This is of course all dependant on situations as i said.
In certain situations it's better to have low HP and other it's better to buff it up.
If you need to do a lot of sacrifice and you have a way to regain the health lost it's best to go with a lower HP. sicne it's a percentage you sac and not a fixed amount. 20% from 480 is a lot harder to recover than 20% from 180 for instance.
Skills like Demonic Flesh (Blood Magic) or the Warrior's Endure Pain give a temp boost in HP.
Also there's loads of monk enchatns that can boost your Hp up to an insane amount. max Hp's of 4000
