What helped me, when i got started, was a hint another player gave me: Take a quick glance at the Party HP bars every now and then, while keeping your tank as focus target. Since, for the most part anyone but the tank doesnt need much healing anyway, you can just let natural hp regen do its work unless you see a lot of white bar missing somewhere. With the tank, you can just take a guess about how much he generally looses during any encounter and adjust accordingly.

I dont know if that helps you, but if you played MOBA games like LoL for example this might be a familiar playstyle.