There is no fixed flow that is recommend as of what I know as long as the biopellets as tumbling to prevent it from clumping together, which will result in H2S(Hydrogen Sulfide) which is poisonous to the fishes and this is produced when the bacteria are in a anaerobic condition.
GFO = Granular Ferric Oxide, it is a Granular form of Iron based Phosphate remover...
With normal test kit, it always appear 0, becos the lowest a test kit can go in 0.25ppm where as PO4 can exist below 0.25ppm, which is quite high for SPS tanks... This precise measurement can only be measure with a PO4 meter.. Do a search you will find it...
BP is used to reduce Nitrate and small amount of Phosphate by providing food source for the bacteria where they will use up the carbon in BP and at the same time break down Nitrate and Phosphate... So it is advisable to use it with GFO or any phosphate remover...