My understanding is that waferboard is just a cheaper version of OSB, and not something that I would choose as a subfloor. If you were installing ceramic tile, I would probably tell you to add a layer of 1/2" plywood and start tiling. However, you're planning on installing natural stone, and a very soft one at that.
Since two layers of plywood/OSB are required for stone, my inclination would be to remove what you have there and go back over the joists with 3/4" plywood followed by a layer of 1/2" plywood. Both layers should have a face grade of A, B, or C and have an exposure 1 rating. The 3/4" ply should be glued to the joists with construction adhesive, and secured with screws or ring shank nails every 8". The layer of 1/2" ply should be installed with screws or ring shank nails only, no glue, using the recommendations in
this article to install it. That will give you the best chance at a successful installation, assuming your calculations on the joist deflection are correct.