How can you tell how much you should eat?

Most of us eat according to habit or compulsion, not according to how much our body needs. Most people have no idea whatsoever how much food their body needs.

There's a Japanese saying, "hara hachi bu," which means "eat until you're 80 percent full."
But most people don't pay attention to their stomach as they eat.

So try recording how full your stomach is before and after you eat each meal. Just put your attention there and feel the sensation of food in your stomach. At first, you won't have much basis for judging, but after a while, you'll be able to use your previous sensations to get a better sense of it.

Then wait half an hour and feel the stomach again (with your mind, not your hand). Try to put your attention on your stomach several times throughout the day, for a second or two each time. Soon, you'll be much more aware of your stomach and how full it is. You'll notice when you're full and won't be as likely to keep on eating anyway.