The rotation changes entirely. In the mastertfrost priority, your main skill is howling blast and you only use obliterate when your unholy runes are about to cap or when the second rune is about halfway when you have a KM proc.
You also need to make sure you go all the way to the spell hit cap.
Spell hit cap is not necessary at all as only ~30-35% of masterfrost damage relies on that, when your extra 200 hit rating to get spell hit capped could be better spent on mastery etc.
You basically use Howling blast instead of Obliterate, unless you have the second unholy rune coming up, or when you have very high resources and KM has procced etc, then you Obliterate. So all you need to do is to keep one unholy rune up at all times, so when you Frost strike you get the other 2 empty runes to proc for more Howling blasts. Obviously use Frost strike when a rune is completely empty, same as hastefrost.