User blog comment:Varnagas/AP scaling per second/@comment-3450702-20120828133826

The difference between casters and physical DPS champs is that casters have much higher burst. Even if their magical DPS is only 1/3 of the AD carry or even lower, their initial burst is often enough to cripple a target enough to get him out of the fight completely.

I'd use this for the following AP DPS champions:

Rumble, Karthus, Cassiopea, Zyra (althought it'd be complicated for her), Amumu, Heimerdinger, Malzahar, Orianna, Vlad, Xerath, Swain, Gragas and perhaps a few others.

I would not apply this calculation to the following high burst-high cooldown champions:

Ahri, Annie, Veigar, LeBlanc, Akali, Cho'Gath, Sion, AP Master Yi and also a few others are not very good picks. The reasoning is, you will not get a reliable amount of their spells in a fight. Example, you might get 1 or 2 Dark Matters with Veigar, but it's a huge leap if you get only one or also squeeze in the second, cooldown regardless. With the previous champions, their cooldowns are usually low enough to reliably spam.