Dan

I concur once again. If the plywood isn't "18mm marine ply or similar" then it has to be the expensive fastflex route. If it is 18mm and screwed at a minimum of 300mm centres with noggins where there's nothing to screw into on the underside then you could use a two part cement based adhesive or there are a few single part adhesives that should cope from the odd adhesive manufacturer, although I think I'd over-engineer if the tiles were not cheap.

From a cutting point of view the more times you scribe on certain tiles (elthough maybe not porcelain if it's fully vitrified / full bodied) the more chance there is of the tile breaking over all lines and possibly causing it to snap the tile to a shape nothing like the shape you need.... usually cutting 90% of the tile where you want it and the end away from the breaker usuall just sort of shots to the left or right so sometimes you can nibble the bit off it's left on and you can use your cut but sometimes it will shear off into your cut and render it useless.

So the less scoring the better but if it's not breaking the first time I suppose you could give it another scribe but dont break too many tiles before opting for the wet cutter as it will give a much neater cut on a stronger tile anyway and if you have time it might be worth just using the wet cutter anyway for that reason.

And like Mr McP I hate seeing a cement based white grout in a floor grout joint. Epoxy yes but that's a different ball-game alltogether. Use grey as it's neater and more consistant. And you'll be using a Wide-joint grout for your 4mm+ grout joints.