Fireball support help

There is also a different method.

You can also do it more "simply," but with more work involved (IMO). That is, more work, less thinking. Instead of starting from tooltip DPS, you would start from tooltip base damage. From there, you multiply:

(Minimum damage + maximum damage) / 2 ×
1 / 0.85 ×
(100% plus sum of all increased cast speed) ×
(100% plus sum of all increased damage on fireball) ×
(100% plus the product (crit chance % × crit multi %))

The 1/0.85 comes from the base Cast Time of Fireball, which is 0.85 seconds.

EDIT : Note that multiplying by 123% for example means multiplying by 1.23