I meant to keep it at 1 shift per extra round. How about making it straight up that the wizard has to roll Discipline each round to direct the attack, whatever the target is? And if the wizard decides to do something else, the energy either doesn't hit anything (if he has more duration and wants to use it in a later round) or can be released as fallout?
Something like...
Harry is minding his own business when he's beset by three ghouls. He wants to conserve his mental stress track, but at the same time he doesn't think he can hit all the ghouls in one round if he divides his attack roll. So he throws 6 shifts of power into summoning up a Weapon:4 fireball, with 2 shifts going toward duration for a three-round attack, hoping to take each out one-by-one. He rolls his Discipline to control it the first time, and rolls well, scoring a 6 to control the energy, and the first target blows its defense roll, getting only a 3, taking a 7 shift hit and going down for the count. The next round, he throws it at another ghoul, but this ghoul rolls better and takes only a 4-shift hit. In the process, though, he's taken a couple hits himself, and decides that he's going to need a shield to survive, so in his third round, he throws up a block--but since he's not directing that fireball anymore and it's on its last exchange, he agrees to let it go into the world as fallout.
The reason I divided it up between mobile targets (enemies) and stationary ones (zones, objects), is because the latter aren't moving, and to me, logically, if a wizard creates a room-wide firestorm for three rounds he should be able to just leave it where it is, indiscriminately torching whatever's unfortunate enough to be in that zone--in this way, it's treated like a block almost--while if he's trying to chase down individuals with a single, constant fireball, he should have to actively direct it.