Below is a break down on how to Calculate Spell Damage. (Courtesy to Beimeith.)
Spell Damage also uses a wonky formula:
Using a float value "x"
1) Find which is higher Refresh Time or Recast Time.
2) Add whichever is higher from 1) to the Cast Time of the spell to get the Total Cast Time
3) Use one of 3 formulas to find x:
4) Multiply your worn spell damage by x
5) Truncate the result