That sort of timing is really tight though, and if you're off by even a little bit, the heal will be wasted before the boss attack finishes or you'll be interrupted despite the cast bar reaching 100% (damage-based interruptions don't have to honor that end-bit of cast bar that you can cheese in most cases). Might as well Surecast if you have it, and there aren't so many useful cross class abilities that you're compelled not to take it.