Run you joist dimmensions through the "deflecto" in the blue bar above...you'll find your floor is fine for ceramic, but not stone. rethink your choices. With ceramic, you don't need the second layer of plywood. You don't need or really want the felt between the layers, either. You'll need either a support wall under the floor somewhere or to reenforce the floor somehow.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters),
Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.