I include the amount of the serving in the description of the food. You can make 100 mL of your custom food the serving size, and if you eat more or less of that serving, adjust it. For example, if you eat 150 mL of that food, it's 1.5 servings, 75 mL is .75 of serving, etc.
Starting Weight: 150.4 (2-5-13)
Current Weight: 139.5
Goal Weight: 130 (5-31-13)