Simply: do not. While it is technically a gain to cancel a Frostbolt cast if you can hit it in the first 750 milliseconds of the cast once you get Brain Freeze at 5 Icicles, the actual gain from it is so minor as to be almost unnoticeable. We cannot foresee any real situation in which this is an optimization even top end players attempt to do. All effort at attempting to min-max this small of a gain should instead be spent on optimizing your normal gameplay, as you will see larger gains out of that, be it through minimizing movement or getting more comfortable with the flow of a fight, allowing you to plan things out like how to AoE more effectively, or simply minimizing your downtime.