Pour a measured cup of paint into a container that can hold at least 2 cups.
Add 1 tablespoon of paint thinner to the cup of paint.
Stir the paint with a paint stick. Scrape the bottom and sides of the container to make sure you mix all of the paint.
Paint a thin streak of the thinned paint onto a piece of scrap paper or cardboard to test its thickness. Wait a few minutes for it to dry to ascertain true coverage and color.
Repeat Steps 2 through 4 until you reach the desired consistency. Write down the number of tablepsoons of paint thinner required.
Calculate how much thinner you need to thin the main batch. Most paint comes in quarts or gallons. There are 4 cups in a quart and 16 cups in a gallon. Multiply the number of tablespoons you added by the number of cups remaining in the container (minus the cup you already took out). There are 16 tablespoons in a cup.