I think that is quite possibly the kind of rules dissection that they want to avoid for dndNext... The whole "rulings, not rules" thing.If I was a DM I would decide wholly on a case by case basis, and if I were a player I'd hope that my DM would do the same thing. Heck, if a player was by a corner and said "can I use expertise dice to reduce the fireball/dragon breath damage by ducking back a little I'd say "sure!"
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