It could also just be easier to start with the existing RAW (specifically, the reshaping magic sidebar). If you are starting with a Shield that is:
a) strong enough to stop the incoming spell, and
b) has enough duration to last, then
c) you can end the spell early on the following round in order to redirect that energy.
The problem with (also, the desired effect of) spell reflection is getting something for free or incredibly cheap. In other game systems, spell reflection is a pretty powerful spell or ability. What folks usually want is one or more of the following:
a) an instant counterattack
b) preferably one that automatically hits
c) no personal energy expended
d) no personal skill involved (acts like a mirror)
As such, perhaps a GM could simply charge a premium in shifts for one or more of those advantages, on top of one's Shield spell. Or let the player pay those shifts out of the spell he's burning.
So, let's say Terry Preston has a Shield 8 up, with two rounds of effect (a 10-shift spell!), when Jerry Clanston attacks with an Evocation spell that only manages to scrape the shield (a 3-shift spell, and a Discipline check of 4 for a combined effect of 7 shifts - not enough to bypass T's Shield). T could opt to (for example):
Sacrifice 0 shifts of his Shield spell to simply counterattack next exchange with an Evocation:8 effect.
Sacrifice 2 shifts of his Shield spell to buy the ability to counterattack right now (moving one up on the Time chart).
Sacrifice 3+ shifts of his Shield to buy the Aspect "Surprise Counterattack" which he can tag to drop J's defense to 0.
Sacrifice X shifts to get other benefits.
So, T could instantly counterattack by burning his 8-shift Shield spell this exchange to get a 6-shift Evocation, and a 2-shift premium allowing him to attack this same exchange.
Or, T could have a better chance to hit by burning his 8-Shield spell next exchange to get a 5-shift Evocation, and a 3-shift premium to buy the Aspect "Surprise Counterattack" which he can tag to drop J's defense to 0.
How does this look?