It is not based on gallons.
You mix up your nutes in a gallon jug and then add a little at a time until you have reached your desired ppm in your reservoir.
You need to have a meter to measure the ppm, so if you don't already have one, buy one ASAP. Also,
@StinkBud is the man and the master, but he likes to run his nutes a little hotter than I do personally (I am am a cheap ass, lol). Anyway, I run my nutes closer to the 1000 ppm mark. I also run a slightly different ratio of the nutes and a few additives with great success.
750ml PBPG, 250ml CalMag, and 200ml Humic Acid (LK). Before I start adding my base nute solution as mentioned above I add 2 ml per gallon of Hydroguard which will help keep your roots extra healthy and I add 2ml per gallon of Silica Blast which helps keep my stems super strong and healthy. After that I start adding my nute solution a little at a time until I get to around 1000 ppm. I have never had any signs of underfeeding with any of my strains at 1000ppm.
When it comes time to flower I add 250 ml of sweet to the recipe above and replace the PBPG with PBPB (bloom). I also increase the Silica Blast to 4 ml per gallon.
This works for me pretty damn well, I am sure there are better formulas though, eg the
@superstoner1 formula that many many have had great success with, jut a little too complex for me personally.
Holler at me if you need any further help. I have been using the SB method much longer than I have been active on RUI.