I've found that I've been forced to be mobile as the white mage and finding that positioning that let's you get that cast in without getting caught by avoidables is key.

I've tended to use Swiftcast more liberally instead of only Raise scenarios.

Same with Divine Seal, it'll usually go up alongside with a Medica II, a Regen on a non-tank, or try to get a Cure II out there over the MP friendly Cure.

If party members are taking too much damage from avoidables, I consider this outside the realm of healer's correction. (I.e. You can make up for the missing knowledge or mistake of one player on occasion, but not for two players or lots of mistakes).