Brewing Hibiscus tea
I use 1 rounded teaspoon for 12 to 16 ounces (depends on the mug I'm using), so about 8 teaspoons (or about 2 1/2 tbsp) for a gallon of water. Hot water, just under boiling, is best.  Ideally, the water is heated and poured onto the tea at just under boiling.  Because I don't think you want to boil a gallon of water, I'd use the amount of hibiscus for a gallon of tea, but only use about a quart of hot water to start, let the tea steep in that (at least 5 minutes, longer will give a stronger flavor) then add the remaining 3 quarts of water.