I think you're mixing me up with Dave there. Happens a lot to be fair.
I can perhaps answer the question though mind.
When setting out a floor you need to take into account your four common walls (there might be more if there is a lot of boxing in etc), any bath that might have bathpanels that end up with a square floor plan. The door way.
Then the loo if that ends up touching the floor (not all do these days) as you might want a grout line in the centre of that.
When you're doing a wetroom area in a large room that might not all have water running over it, you would perhaps section off the sloping area that falls into the trap, but you would still want the grout lines marrying up with the rest of the room as you wouldn't want to look like you completely tiled it separately.
The trap would either want a grout line down the centre of it in both directions, but it could also be in the centre of a tile, though I have seen them with a grout line running only one way through it and the other way it has been centralised in the tile (so you've effectively cut two half moon shapes out of a tile to fit around the trap).
You would make a guage staff rather than dry-laying tiles. So you draw on a 2inch wide piece of woof thats about a meter and a half long all your tile widths and lengths (one side for each) taking into account your spacers. Then you can use this to guage where all the tiles are going without laying a single tile dry. You then mark the floor, and when you're happy with it, use a string line to snap a line work to which keeps you on your set-out perfectly.
Dry-laying tiles is a bit DIY to be honest.
Once happy with it, you'd do about a square meter at a time. Spread the adhesive, fix the tiles, wash the whole area off, check you're still inkeeping with the setout, then do another square meter or so.
When you walk in a room you should not see any 'slithers' of tile. So when you're setting out the aim is to ensure that you have the most appealing look to the eye, when standing in all the common areas. So you're moving the 'grid' of the tiles around in your mind effectively until you are happy that you end up with the biggest sized cuts in all the common areas I mention in the first paragraph.
The way you suggested doing it would make the trap look lovely but then if you ended up with 15mm cuts at either end of the room then you could have done with moving the set out so that the trap wasn't in the position it is in but half a tile along. So your 15mm slithers then end up half a tile bigger, and the trap still looks lovely but now so do the sides where it meets the wall or skirting or whatever.
Setting out is key to happy and stree-free tiling that ends up looking gorgeous. So make sure you cover that well in your tiling guide. The rest is just keeping to the setout then and making sure the product selection was spot on.
The biggest balls ups that we see are wrong setting out, wrong adhesive and grout used, or incorrect fixing (so dot-and-dab or spot-fixing tiles rather than spreading the adhesive out well to give 100% coverage). On the jobs where all three apply it's fair to say the tiling needs to be ripped up and re-done.