Im 3 years late, but Ive made a javascript calculator for multiple panel wings in
http://wingcgcalc.bruder.com.br/
MAC for single panels is simple, there is a well know graphic method, and when your wing has multiple panels, you find the individual MAC's for each panel and use a 'general MAC' using the weighted (by the area) mean of all MACs. See
http://www.paracreo.com/articles/Par...ic%20Chord.pdf for the math details.
Or use my javascript calculator