OK, the flat hone guy's opinion: Take thick glass plate 35/35 cm (get it at your locall glass suply), stick a sheat of 800-1000 high quality sandpaper with double stick tape, load it with water, and start lapping the stone with 8-figured motion. before that, you can draw with a pencil parralel lines to both surfaces of the stone. When they are gone, your stone is "flat". Now, the point I of this action is more to smooth the factory surface then to flatten the stone, which might be a bit coarse for razor use. It will check the flattness of the hone, too, so you are all set...

Nenad