Welll...you mix up some nutes according to instructions on the bottle (sorry..not familiar with FF stuff) or preferably at like half or 3/4 recommended strength and give them as much as they need. Depending on your medium and method. If it's soil or soiless (sunshine, promix, coco...) you give them enough to saturate the medium.
No one can tell you if that's a gallon..we don't know the size of your "buckets" or medium. Like...if you're using 12oz plastic cups a gallon is too much. If you're using 45 gallon drums as plant pots...a gallon isnt enough.
The question you're asking is like saying...is one pound of butter enough for my potatoes? Depends on how many potatoes you cooked and if they're baked, mashed, fries....
Is 3 quarts of oil enough for an engine?? DEPENDS
I suggest you feed according to the directions on the bottle every 3rd watering if you are freaked you'll overdose em.
Feed-water-water-feed/etc..... Adjust yer nutes according to what your eyes see.
use as much water as it takes to see runoff comin out of the bucket whether you use fert or not....
good luck
No. 1 gallon of water with nutes mixed in will probably not be enough for 5 gallon containers. You want to water like Skyhigh said. Water until it runs out the bottom of the container. If it took 20 gallons it wouldn't matter because the concentration of nutes in the water is the same so no matter how much you put in. So don't worry about the amount of water as much as the amount of nutes you mix in the water and how the plant responds to it. I'm guessing it will take almost 2 gallons of water for a 5 gallon bucket. Read sky highs post again (forgot to quote it...)and I think you'll be on your way. Good luck
fred