Unless you can see inside your stomach, when you stop eating is fine. If you want to be fully digested and empty stomach for 16 hours just add some extra hours to your fast. I guess it depends what your goal is.
Fasting = Not Eating. So, the timer starts after your last bite (some might say last swallow). You can’t count the time from when the food exits the stomach. It can exit at different rates depending on the individual, amount and composition of the meal.