Personally I factor in the mana regenerated while I'm casting a spell as well when it comes to comparing the healing per mana. It's arguable whether or not you should do that though, but it certainly makes downranking look a bit more favorable.

How I do so is basically calculating an 'effective mana cost', which is basically the base cost minus the amount you regenerate during a cast. Mana regeneration basically provides a flat amount of mana cost reduction. I'll show you an entirely fictional example:

Spell A has a 2.5 second cast time, costs 600 mana, and heals 3000 health. Spell B has a 2.5 second cast time, costs 300 mana, and heals 1500 health. Without taking into account regeneration, the two of them are equal healing per mana (To be exact, 5 healing per mana).

Now let's say that in this 2.5 second period that you're casting the spell, you regenerate 50 mana. That gets us an effective mana cost reduction of 50... Now Spell A does 5.45 healing per mana, while Spell B does 6 healing per mana.

The validity of regen while casting for these kind of calculations is something you can have long and very complicated arguments about however.

I must be missing something... Are you saying that because it takes longer to cast two low rank spell, you gain more mana regen during this time???

Because in that case, with spell A, in 5 seconds, you should regen 100 mana while healing 3000, so also 6 heal per mana... (and that's not counting that by casting fewer spell, you can have more regen tick oustside the FSR)...

So no, i still don't really see the point...