+spell damage and instant spells

[Skandranon]

So, this is actually a misunderstanding.....

No, it isn't.

I took out my regular damage suit, +351 frost damage, for a spin. Rank one Frostbolts, properly 1.5 second casts, should be getting 43% of the bonus, or about 151 damage. The bonus it was actually receiving was +60, or roughly 17% of the bonus. There is no doubt that there's some penalty for lower ranked spells, though it does not apply to every spell. IIRC, Flash heal has no rank penalty.

One other comment.  I have not tested AoE outside of mages with +damage, but for Arcane Explosion at least, I have not seen any kind of "we assume 3 targets" thing.  By my math, it's been a straight 43%.  My AE hits for around 300 per hit and I have no where near the +327 damage gear that would be required if there were a 1/3 in there.  I haven't really ever tested this for, say, Hellfire or Hurricane, though, so this may be an exception.
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My Arcane Explosion (rank 6) has a listed damage of 249-270. The same damage set up is +284 to all spells. By the 43% calculation, each of my AEs should be receiving roughly a +122 damage increase. Empirical testing reveals that they hit, on average, for roughly 300 damage, which is a bonus of about a bonus of 40, which corresponds almost exactly with DamageBonus*0.43*1/3.