First, I would check to be sure whether your existing 1/2" plus another 5/8" is sufficient for a tile floor. One of the tile sites has a floor load/strength calculator, and maybe someone here can direct you to that link.Originally Posted by marklevinson1
In any case, you should do the ceiling first (after adding another layer to your floor), then push the wallboard up tight to it before fastening, allowing at least an inch (and no less) at the bottom for your *final* layers of 1/2" concrete board, thinset and tile (3/8" - 1/2").
Also, keep in mind that your baseboard moulding is going to cover about 3" of wall above the finished floor. So then, and for example:
96" - 3/4" flooring - 1/2" ceiling board - 1/2" concrete board - 1/2" tiling space - 1/2" breathing room = 93-1/4" drywall cut length for walls.