Alright, see that makes sense.
The way I was reading it was that the total shifts of power to the effect was entirely based on your Conviction, and rolling high was only important if you were calling up more shifts.
What you are saying makes much more sense, and I will plan on using it.
To make it clear, an example of what you are saying would be:
Our protagonist wizard is being surrounded by a gang of three thugs. He wants to disable them without harming them. He creates a gravity surge under them to hold them in place (Earth Magic), using the grapple rules, and pulls 4 Shifts of power into it (3 for Good Conviction, +1 for a Offensive Earth Focus Item in this example).
He then Rolls His Superb Discipline, getting a + on the Roll, for a total of +6. He Invokes an Aspect (It's Worth Doing Something Well) to bring it to a +8, and then has a +1 Focus Item for Control as well, bringing the grand total to a +9.
He uses 4 for the base control to hold the energy of the spell correctly, then 4 more to make the power a spray attack and affect all 3 thugs. Lastly, he uses 1 shift to extend the duration, giving him another exchange, and a precious couple of minutes to get the heck out of dodge.
Does that all make sense?