Mirror is a double edge sword. It can be useful and also be harmful. In your case, the purpose of placing mirror at ur dining table
is to "double" the amount of food u cook. According to chinese traddition, food is wealth. more wealth = more food and vice verse.
to be more extreme, u should construct a platform for dining area. There a chinese saying... having one's meal is same as emperor.
but of course platform in dining area is subjective.
Mirror facing the main door.
Subjective, if there a "bad arrow" facing ur door. Mirro is use to reflect away the bad thing. BUt is ur wealth is coming from the main door, thus having
mirror is also like reflecting away wealth coming to u.
It will be wise to consult a FSM.
Regards