Ah. So I should more or less make my burn phase last as long as arcane power, essentially? I know that's something I'm DEFINITELY not doing - I've been burning down to 50% mana and extending it when I can with supernovas, arcane missiles, and so on. In fights that are a lot of movement like Xul I sometimes NEVER evocate until phase 4 because there's so much movement that I regen to full...
That's probably the single biggest thing I'm doing wrong, then - I really didn't understand that burn phase was dependent on arcane power, not on mana, per se. I'll see how that feels on dummies, then.