Eating half is a convenient starting point, but you could try two thirds or any other reasonable reduction in volume. Worldwide, many people are indeed eating twice the calories they need, so eating about half before the pause is a simple goal. When we eat in a hurry, it is easy to overindulge. We have to reject the idea of eating our usual volume of food just to fill our stomach.
A recent patient described the 10-minute lunch he enjoys four days per week: a double cheeseburger, a chicken sandwich, a large order of fries, and a large diet soda, totaling 1380 calories. Well, he only needs 1800 calories per day, so for him, eating half to two thirds would be a good start.