Hi, the thickness of your subfloor is not the only question you should have...what is the current floor like? If it also ceramic tile and in good condition you may be able to just tile on top of it. If the existing floor has alot of "give" in it you may need to remove it and reinforce the framing. In general you're best off with 2 layers of 3/4" plywood installed directly on srtaight and flat framing. Make sure to offset the seams of the plywood layers as much as possible to avoid seams "showing" through and cracking the tile, and use both adhesive and screws to fasten the layers together. Depending on the height of your finished floor adjacent to the tile floor you can go with 1/2" for the top layer, but heavier is better. For the "transition" (I assume it's a doorway) just run the new subfloor into the doorway and install the marble saddle along with the tile. If the saddle is perpendicular to the floor framing you'd be well off to add blocking between the framing directly below the saddle before installing the subfloor. You may have to trim the door bottom. Talk to a tile man about the best way to put in the tile. I wouldn't put it right on the plywood. Good Luck!