If you are trying to lose weight and tone up then you should never overeat. Overeating forces your body to store the excess and save it for later (fat). So it is very important to watch your portions and not overeat.
Once you feel your tummy getting full it is best to put down the fork. Your body needs to food to suvrvive and any more than is needed will be stored away for later. So keep account of what you have eaten and even if there is still food left on the plate, store it away for next time and save the extra calories from converting to fat.