Mirror To See The Doorway Day 18 Feng Shui

A basic feng shui principle is not not sit with your back to the door. The problem is my desk is a huge L shape and  I don't want windows to be reflected in the computer screen.

I can only have the desk in one spot but the door is behind me. I put a mirror in the corner about a foot above my head and now I can see at a glance anyone who walks in.