TC, don't be afraid to use Medica II, especially if you want to DPS. Don't think of it in terms of overhealing or healing party members who aren't taking damage, think of it as an extra tick of healing on the tank; use it together with Regen to make your life easier and give you time to DPS or deal with mechanics, or even just to supplement your healing during big pulls or heavy damage phases.

I ALWAYS open normal pulls with Medica II + Regen on the tank, I usually save Assylum unless it's a really big pull. Don't worry about overhealing, but don't worry about keeping the tank above 75%, either; you're a White Mage, your specialty is BIG heals, so let their HP drop while you squeeze out another Holy or finish dotting all the mobs, and then hit them with tetra or bene or a couple of Cure IIs just before they start to panic. The occasional reminder that you hold their life in your hands is good; keeps the tanks from getting too cocky ^_~.