A pakua mirror is used to ward off negative vibrations entering a building mostly through the main door. The wrong or harmful Chi will be emanating from a distant point due to secret arrows. It may be an ugly looking tree in front of your house or roof line of the opposite building or the cross of a church or a temple tower.
Secret arrows can get formed at great distances and the house can still be receiving bad Chi although you may not find any source of secret arrows in front or at the back of your building. It is therefore always safe to install a pakua mirror on the front door.
If you have a back door to the house then it is advisable to have one more pakua mirror installed over it so that both doors are safe.