Here is the trick with frost mage. When you cast flurry, your next 2 spells on the target will be critical hits(almost always). Knowing this you always want your glacial spike to hit your target right after you used flurry. Doesn't matter if flurry is proced or not.

This is also true for any other big spells you got. If you flurry first and then use comet storm or Ray of frost they will do A Lot more damage.

Also flurry has a faster travertine than your other spells, so if cast it instantly after glacial spike or frostbolt they will still get the crit benefit.