How big is the sweet spot for the vandersteen quatro wood CT speakers?

Just fell in love and purchased a pair of Vandersteen Quatro Wood CT speakers. Going to be set up in my living room approximately 13 x21. The speakers will be 13 ft apart. We'll have two chairs. Our heads will be 6 ft apart  and 18 ft from the speakers. Has anyone had any success creating a sweet spot  that wide (so 2 people are in it)?
The 6” window is with respect to vertical distance. The horizontal “sweet spot” is actually pretty wide as the horizontal off-axis response is quite consistent and smooth. 
@miketuason ,
If I were you, I would try moving the speakers. Sometimes the formulas don't work and placing where the 'rules' say no, may make a difference.
If you can find a Vandy dealer nearby, or call Johnny at Audioconnection,  you'll get more information for free than you can imagine. 

Johnny R guided my cloth Quattro set up, ( purchased from Audio Connection, shipped to California without incident).  They are pushed into the corners and image outrageously. I've read that the Wood CTs are tuned to work best further from boundaries than the cloth.