Just a question, but, if you count 12 before the snap and the snap goes off before you can signal what you've seen, wouldn't it make sense to shut down the play and then verify after the snap? If you've counted wrong, there's no foul-no harm, you pick up your flag, advise the referee and move on.
The fact that your reaction, although perfectly justified, may have been a tad slow shouldn't increase the penalty for something you observed before the snap. If you weren't sure of your count, that may be a different story, but when you count 12, and the ball hasn't been snapped, that's a dead ball foul, whether you blow your whistle in time or not.