I have been making a simple sea salt scrub for myself and friends for a couple of years. Recently one of my friends asked me to make her a few jars to give to people she works with. Since I don't know them and how they will take care of the product, I decided to use a preservative. The one I chose says to use it at .25 to 1 %. My question is since my recipe is just basically the sea salt 8oz, oil 4oz, and an essential oil at 10 drops. Should I only figure the percentage based on just the liquid or the salt and liquid? Sorry for the stupid question, but I am new to using a preservative.