SFC Grant Singley (USA Ret), may be the best service rifle shooter in the past 25 years. A multiple National Champion, Interservice Champion, and President's 100 Champion, attached is a link to an article from 2007 written by Singley about his crossed ankle sitting position. His positionworks for some shooters better than other crossed ankle positions. But the only way to know if it will work for you is to try it.
http://www.odcmp.org/0307/?page=USAMU_CASITTING