The Cardas website has interesting advice for speaker/listener placement. They heavily rely on Fibonacci ratios for room dimensions and speaker placement with a mid-field setup. As always generic advice should only be used as a starting point, after that let your ears be your guide.
I have a Fibonacci room (8ft x 13ft x 21ft) and used the Cardas advice as a starting point for narrow wall mid-field setup (speakers centered 33.5 " - 0.85 meters from side walls and 55.5" - 1.4 meters from front wall).