When you say you're off, what do you mean? too close or far from the butterfly? both have issues of their own, but frankly, I wouldn't worry about re-cutting that pretty groove (man, that's REALLY nice work, the groove and the IAC channel) until you've driven it; it looks to me like you can get in through the IAC hole to plug that with a dab of epoxy mixed with the aluminum shavings. A wee little patch won't disrupt what a nice full groove can do, and may even make it's effects greater!
Ron's said frequently that Gadgetpeople have to be brave and nimble when approaching problems to be solved. look at the "problem" from another way: see if you can come attack it from behind! You may be surprised at just how much room there is back there.