Wireless power transmission calculations
Here are some app notes related specifically to this topic that may help:

http://cds.linear.com/docs/en/application-note/AN138fc.pdf

http://www.digikey.com/~/media/Images/Product%20Highlights/W/Wurth%20Electronics%20Inc/wireless-power-solutions/wurth-wireless-power-charging.pdf?la=en-US

and here is one with all boring the theory:
http://publications.lib.chalmers.se/records/fulltext/175567/175567.pdf