+spell damage and instant spells

[TheWesson]

The 3.5 second rule is really a rule of thumb. It doesn't always apply.
Not always true. Some instants get no bonus, or a different bonus fraction.

True.  Talents improving casting time do not affect +damage bonus.

Not true. The actual bonus varies spell to spell, and in previous patches, DoTs were completely ignored for +damage.

Immolate, a 2 second Burst then dot warlock fire spell, gets 20% of the bonus for its burst, and receives 57 percent of the bonus to its dot. Odd, no? IIRC, at one point, soul fire (6s cast) received zero bonus from +damage.

You must look for empirical +damage result charts or test these out for yourself. The 3.5 second formula simply doesn't always work, and there's a lot of balance changes done in patches both listed and unlisted.
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what pure instants (no buff/debuff) are you thinking of ?
what pure DoTs or HoTs (no DD) are you thinking of? (I know, Renew is broken, or was, on other-targets.)

I'd like to know how good the rules of thumb are.

Thanks ...