Sometimes you fail at the counterspell assessment, but you pretty much always know OOC how much Power is in the spell you're gonna counter. So then you're stuck with "well, he isn't sure, so he does something else." Which has never quite felt right to me. So, I came up with something on the fly during a scene tonight...use or don't as you like...
If your player wants to
commit to the counterspell with a "guess" after the assessment failure, roll the dice. Add the result to the power of the spell in question, and that's how much Power they 'guess' and will put into the spell. So if the spell you're going to counter has 6 power remaining in it to negate it, and you roll -2 on the dice, you only guess 4 Power and your Counterspell fails--but you've committed; you HAVE to cast that spell at 4 power for your action. If you roll a +4, you'd put in a whopping 10 shifts of power...Whatever happens, you then cast the spell normally.
It's risky, but you do have a statistical advantage in getting a 0 or better means the spell vanishes...though you as a GM may have some excess Fallout to play with
And compel opportunities, of course. Best roll you want is a 0, which you have ~1/4 chance of getting. Too little, and the counterspell fails (or possibly the spell gets supercharged). Too much and you might supercharge the spell or make it explode and send the excess energy outward.
You could invoke Aspects for boosts or rerolls on this "guess" roll as usual. The main disadvantage to this method is that after the assessment, there's another roll, for a total of 3 rolls to do one action. But, *shrug*, it's rare enough of a thing to not bother me there.