With power in Evocation, technically speaking, can't you draw as much power as you feel like taking stress for by default? The invoke on control is there so that you can draw and take stress/consquences for big ass spells then control said big ass spells. At least how I've taken it.
Well, you could, say do a 12 shift spell, for instance
Example 1: Using tags for control
With conviction 4 that would be a 4 box (for 7shifts)
Then a moderate (11 shifts)
Then a minor (13 shifts)
You could, then, spend all your tags to try to control all that power.
Example 2(using tags for power)
12 shift spell:
4box (7shifts of power)
Tag camp fire (9 shifts)
Invoke Wizard aspect (11 shifts)
Tag Summer Solstice(13 shifts).
In example 1, you took a minor and a moderate, but probably controlled the spell
In example 2, you didn't take any consequences, but now you have no more tags to control the 12 shift spell.
I'm not sure if you're allowed to do example 2. That's what I'm asking.