Calculated field for Grand Total

Okay, I can easily do this outside of the pivot, but I'm really interested in learning how to do this through a calculated field. In the attachment I have a pivot. In my columns I have EA Elig, which has two value fields, EA and Non-EA. What I'd like is a calculated field showing EA/SUM(EA+Non-EA). When I try to do it as a calculated field "EA Elig" shows up as a field I can use, but "EA" and "Non-EA" don't. Any help would be greatly appreciated.