If you just mix up the brine and don't put any cucumbers or other vegetables in it you can just boil it and use for the amount of jars you have, then refrigerate the rest of the brine.

That is why I like the Heinz dill pickles I make. It only makes about 5 pints and if I have extra brine I can keep in the fridge. That is my favorite dill pickle recipe.

You need the same amount of vinegar or more vinegar to water ratio, not double vinegar. It is usually equal amounts. This is for cucumbers. Other vegetables may need all vinegar and no water.

The mixes have citric acid added to them.

Agree. It is the citric acid already in the pickle mixes that make it ok to use the amount of vinegar and water called for when using the mixes as long as you strictly follow the directions on the package. Don't add other ingredients, don't skip steps, and don't make-up your own mixes.

Mrs. Wages says on the package that left over brine can be stored in the fridge for a few days. Just put it in a jar with a lid. But note that it is then re-heated back to boiling before the second use.

Anyway, I keep reading that you need to use twice as much vinegar as water (at least) for the recipe to be safe

No, don't mis-understand what you are reading. It is the ratio of water to vinegar that is important and the vegetable ingredients in the recipe. 1:1 water to vinegar is the minimum when there is no citric acid and only cucumbers. 1:1.5 water to vinegar is common in other recipes. 1:2 water to vinegar (2x as much vinegar) is called for in some recipes especially when other ingredients like onions, peppers, garlic, etc. are added to the cucumbers.

Dave