I always have stoneskin on the tank between pulls, when the enemies are about to die I start casting it. Eye for an eye on big pulls, divine seal if its a big pull, top off tank, put up regen, swiftcast holy, then PoM holy.

As for when to stop...I just watch the tanks health (as you stated) and heal when I think it's nesecary. It's just kind of something you judge and know when to do...hard to explain lol. Something you could do is save your swiftcast or pom until you feel you have the timing down better.
Also, I don't know about the medica II thing...holy already takes a ton of mp, and medica II is never something I felt nesecary to keep the tank up while I dpsed