All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You should eat as per your need, taking into account not to go beyond it. If any food remains, you can eat it sometime later.
As for emptying the dish of food, it is a good matter, taking into account that you are not obliged to do so like the dishes of others. In addition, you are not required to eat the food in the kitchen sink or in the recycle bin. However, you should advise those who leave behind some mouthfuls in their dishes to eat them and not to leave them for Satan, especially if eating them would cause them no harm. Besides, if there is a chance to collect all of these mouthfuls to eat them at sometimes later, then this is good.
Allah knows best.