How can I have 13.5 servings left in my FuelPod?

By using the App to dispense FuelPods you have control over the strength of each dispense

A dispense is (1) one 2ml serving of your FuelPod into your water. Each FuelPod contains 15 Standard Strength servings. However, if you dispense a "light" serving you dispense (.5) half of one serving and if you dispense a "strong" serving, you are dispensing (1.5) one and a half servings.