Mix up the gallon kit according to the instructions.
Take 1/3 of the gallon and mix with water to one gallon; this is your working solution.
Take the remaining 2/3 of the concentrate and mix with water to one gallon total volume; this is your replenisher.
For every 30 8x10 prints you make you replenish 300ml.
After your printing session and replenishment you top up the working solution to a full gallon.
Store the replenisher in an accordion bottle or find a way of storing it in a container with as little air getting to it as possible.
When you have used up your replenisher you must mix a new batch. But instead of using water to mix the new concentrate into working solution, you use the old developer - this way you get the benefits of your replenished batch in the new working solution as well.