Tips for Eldritch Battery?

So i was thinking about picking up Eldritch Battery for my 74 fireball witch, but are there some guidelines before doing it? For example, an ideal amount of HP to aim for? What about armor/evasion?

Depends on how you intend to use it; if its just for a little mana more then just stack armor and a little es on sub pieces for the boost and play it the same way as any non ES build. You should also consider using a shield for the additional defenses.

If you intent to go super high ES to get loads of mana to run say a 200 mana/s Arctic Armour all the time then you can go lower HP (~3k should let you survive just about any hit as long as you have over 5k armour and didn't skimp on resists).

Otherwise if you keep resists capped including chaos you can get away with 3-4k hp by playing it safe but beware of Rhoas, Cinder Elementals and Goatmens.

For EB the amount of HP or armour doesnt matter at all. (at least for my spark templar, EK-witch, ligthning strike dagger shadow and my ice spear shadow)

I usually dont wear full ES stuff, only full armor or hybrid stuff (or full evasion for some builds) => I dont need my ES as defense anyway -> I get EB as soon as I can (in generall first some dmg nodes and mana reg, then some life nodes and then straight to EB)