I'm saying it doesn't matter when you add something, as the Power part is a done deal before any dice hit the table.
You decide on a power amount, and calculate how much stress that will cost. The Power you draw can be absolutely any number you want. If however it exceeds your Conviction + Power (+ invokes, it's not 100% clear if the rules allow invoking aspects here, but I think most allow) it costs more than the single mental stress.
After that is complete, and you have decided how much power you draw (whether or not that exceeds your "almost effortless" limit), then you try to control that amount of power.
You have to look at the spellcasting as two separate "rolls", even though you only actually roll dice for one of them. First you set the power, then you control it. Once you move to trying to control the power, the amount is set and won't change.
Evocation example:
I have Good Conviction, Fair Discipline, a Power bonus of +3 and a Control bonus of +2.
I decide to draw 7 shifts of power. This exceeds my Conviction (3) + Power bonus (3) by 1, so it will cost 2 Mental stress.
Then I have to control the 7 shifts. My Discipline (2) + Control bonus (2) adds up to 4. I still need 3 more from the dice and invokes, so I'd better have a few FP saved up.
For thaumaturgy, the only real difference is that the stress cost is one lower, since you don't pay anything up to you "effortless" level.
Does that clear it up?