I feed green water to my 80 gallon reef about every other week and usually a cap full or two!
I prefer home made "mush" to feed my corals and clams! I use raw shrimp, clams, oysters, raw fish, nori, etc and grind it all up in a blender with RO/DI saltwater and flatten it out in a freezer bag and freeze it and break off a quarter size chunk and feed the tank. I am sure the four clams I have and corals love it.
I mush some of the frozen mush so the saltwater looks milky and then feed the tank.
Don't you hate it when that happens? I'm curious myself, as I'm jut getting started with phyto-.
Casey for sure grows his own, in 2L bottles, and I think he said he used a half cup, every other day.
Also, do a search. There was a discussion about this just recently and there is a neat website, (Flame Angel's Reef, I think) that has step-by-step instructions for culture and feeding of phyto- and rotifers.