If the height of your furniture is below 35 inches then an eight to 12 inch space is ideal.
Proper height for dining room mirror.
Arrangements of multiple mirrors on the same wall space should have the center of their arrangement set at 60 to 62 inches as well.
Mirrors placed above headboards and couches require a 5 to 10 inch gap between the mirror and the item.
Tips on hanging a mirror over furniture.
My current home is small there is no dining room just an eat in kitchen area which is small.
I like mirrors in any room of the house for the reason you mentioned it bounces light around the room.
The second companion picture is on a wall that has nothing else on it or under it.
Like all mirrors in the house you should hang it at eye level.
Mirrors reflect light and make a room feel larger and when hung above furniture they act as stunning yet simple wall decor.
The ideal space between a mirror and the furniture below it is six to eight inches if your furniture is approximately 35 inches high.
One is above a 33 buffet which i placed it 10 inches above and it looks great.
I do have a question though on hanging pictures.
I just hung 2 pictures in my dining room.
I miss not having a proper dining room pantry laundryoom but am grateful for what i do have.
I also placed it at the same height as the picture above the buffet.
Hang your dining room mirror on the wall nearest to your table to give the room a brighter more open look and feel.
Mirrors above toilets or near furniture not above furniture should abide by the same rule.