AFAIK, rotes are limited by 'taking zero' in both your power and control stats.
For example,
Harry has Superb (+5) conviction and Good (+3) discipline.
He has an evocation specialization for Fire power
And he has a blasting rod (focus item, +1 offensive fire control)
So without the blasting rod, he normally have 6 power, 3 control for fire
and with the blasting rod, for offensive stuff, he has 6 power, 4 control.
So he could have the following rotes:
Without blasting rod:
Fire attack: power 3
This is as much power as he can automatically control with his discipline. Also, even if he does happen to have his blasting rod handy when he casts it, he only rolls his base discipline to target the rote, since the blasting rod isn't included in the rote.
Fire attack: power 4 to 6
He call call up this much power automatically with his fire power, but he can't automatically control it. So, he would be forced to take fallout every time he tried to cast it. Note that he takes the fallout even if he got lucky on his discipline roll and happened to get enough control to have controlled the spell as if he were casting it normally, since he's using the rote.
Note that there is an exception here: if you invoke aspects on your discipline roll and now have enough to 'take a zero' and still control the spell, you can avoid taking the automatic fallout.
Example:
Fire attack: Power 5
Normally, Harry's good discipline would leave him looking at 2 fallout here, but if he invoked an aspect to add to the discipline roll, he now has enough discipline to automatically control the rote, so can avoid the fallout. Also, he gets the to hit bonus on the discipline roll to actually target the spell.
With blasting rod:
Fire attack, power 4
He can automatically control more power when he has his blasting rod, and he gets to target this rote with his improved control. The disadvantage is that if he loses his blasting rod, or gets it knocked out of his hand, he can't cast this rote at all.
Fire attack, power 5 or 6
Again, a rote that forces Harry to take fallout every time he cast it. But he'll take less fallout and hit better since he is using his blasting rod.
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Since the minimum stress you can take is 1, operating blow your normal power total doesn't help any.
A proposed house rule that seems to be OK is that if you 'spend' 3 shifts on your discipline roll, you can reduce the stress of the spell by one. So with this house rule, someone with superb (+5) discipline could throw power 2 spells around all day (as long as they rolled well on discipline, anyway). Of course, he probably wouldn't be hitting anything with them, since he's giving up 3 shifts to hit too.
