When calculating drain it is based off of the Force of the Spell, which you choose before you cast it.
The Hits you roll determine the results of the spell, limited by the Force, and whether the drain is stun (hits equal to or less than your Magic rating) or physical (hits more than your magic rating).
Also, I loved that last IC post and surmised from it that you were doing exactly what you did. An excellent IC representation of the mechanics!