It's a little more convoluted than above, but only in the sense that it makes the stat even worse.
Recast=max(Base Recast, Base Refresh) |||| (which recast value is higher?)
Total Cast time = (Base Cast + Recast )
If current_level-spell_level>10, dmg multiplier = 0
If (Total Cast Time < 2.5), DMG Multiplier=0.25.
If (Total Cast time >2.5) & (Total Cast time <7), DMG Multiplier = (0.167*(Total Cast Time - 1)
If (Total Cast time >=7), DMG Multiplier = (Total Cast Time / 7)
Additional DMG = trunc(DMG Multiplier * Spell DMG Stat)
When spell damage first came out, it was fun because a lot of the spells that trigger off base spell casts had dummy 100 second cast times, which artificially increased the multiplier those spells receive. Rangers and Druids had very strong benefit from chain casting their Summers lines at that point.