As a rule of thumb, 1 quart water should be given per gallon soil. 5 gallons soil = 5 quarts water/nutrients. Less than this amount and you risk not properly saturating the entire soil mass and forming dry pockets, which creates problems.
I found BMO was excellent when applied to soil with some base nutrients already present. I added high-N bat guano (which they sell periodically and it is GREAT, I highly recommend) and dolomite lime, which really helped keep the leaves green through the last couple weeks. I think as a stand-alone nutrient, the Flower Power is a little weak, and the Cal-Mag deficiencies described above are spot-on...it needs lime or another liquid source of cal-mag.
Combining the Grow it Green and Flower Power in appropriate ratios is a good practice during flower as well.
I don't use their grow/bloom nutrients any longer...I've switched to a potent amended soil mix I make myself (using high-N guano, kelp meal, alfalfa meal, mycos, ~50% worm castings, rare earth, azomite...) supplemented with the General Organics line, and am quite pleased...but I still use the Super Plant Tonic with every feeding.
Because they brew each batch independently, the quality tends to vary from batch to batch. I've had a couple bottles of Flower Power and Super Plant Tonic go bad pretty fast...in fact, my last order they sent me 2 bottles of SPT (I ordered 1), and one of bottles developed a black precipitate in the solution within a couple days and began to smell rotten, so I tossed it.
I think BMO is a great way to start into the world of organics, but not for the organic gardening "expert" (except for the Super Plant Tonic, which I highly endorse).