You will sometimes make some prepared brine to add to ferments that do not have enough liquid to completely cover your veggies. My rule of thumb is 1 TBSP of Natural Salt per cup of distilled water. But a rule of thumb is a rule you use when you have an insufficient understanding of what you are trying to accomplish. Trial and experience, based on the ferment you are preparing, may lead you to use more or less salt. Like a good scientist, I keep a lab notebook to record details and subjective results for each fermented batch.
Brine is also a byproduct of vegetable fermentation. I do not think of it as discard and I hate to throw it away. It is infused with the flavor of the product you were fermenting and is a useful condiment all on its own. I have used brine instead of vinegar, lemon, or lime juice to do fresh pickles of things that do not stand up well to fermentation — like tomatoes and cucumbers. When I am fermenting kimchi and most plump sweet peppers, I keep a small jar on hand to store the excess brine.
Label your brine jars and consider adding some of that brine to your Kimchi Cucumber, Refrigerator Pickles or Tomato Basil Salad. I have also read of people dehydrating brine and using as a condiment to sprinkle on other foods.