How do you adjust if you cannot find 18 oz boxes of cake mix?

Use 1 extra large or jumbo egg, 1/2 the box and 1/2 of all the other ingredients. If the batter is too thin, add cake mix flour until you get the correct batter consistency. Or you can always do the math based on the box you have by dividing the ounces of cake mix you have by 18, then multiplying each ingredient in the recipe by that number. For example, if you have 12 oz of cake mix, every ingredient in the recipe should be multiplied by 12/18 (which is 2/3). So you'd need (2/3)*(1/2) = 1/3 cup vegetable oil, (2/3)*2 = 4/3 egg (meaning one whole egg and a third of another egg), etc.