Given that Astral Flow uses up all of your remaining MP, I think some sort of boost based on how much of your MP you have when you use it would be in order. A whopping 1800 MP fueling a spell had better put out some righteous numbers!

Personally, I would more be in favor of adjusting the MP cost of Astral Flow to merely use a percentage of your max MP - much as SCH's Embrava and Kaustra do. Even a percentage as high as 40 or 50 percent would be preferable to all of it. It would also allow for a little bit of synergy with the new SP move that removes the BP timer.