Wikipedia about THE SWEET SPOT:
The sweet spot is a term used by audiophiles and recording engineers to describe the focal point between two speakers, where an individual is fully capable of hearing the stereo audio mix the way it was intended to be heard by the mixer.
In many studios the imaging guys do not sit in the sweet spot or grab it from the other side: The speakers are placed totally wrong! Traveling a lot and visiting a lot of radio stations in the last years made me aware of that problem. Here is the solution and a control for those, who placed it right.
Learn how to define the sweet spot.
Imagine the middle of both monitors and your head as a triangle. The target is to create a equilateral triangle, so every angle has to be 60° and every side of the triangle has to have the same length. Here’s a picture to clarify.
Make sure, you’ve got the right height with your monitors. You ear has to be on one level with the tweeter of your monitor. Here’s a pic.