As above you try to avoid slithers and unsightly cuts. Your approach would be best made by planning your tiling cuts, and regrettably even though you have a wall that divides evenly in to the 600mm tiles, you have one that does not.
Although I personally think a cut of around 60mm-100mm adjacent to shower screen channel will not be un-pleasing to the eye.
But saying that, that is my opinion only and as the saying goes "beauty and the eye of the beholder", So you may have to consider starting your tiling at the mid-point of the 1800mm wall, with a whole tile centrally crossing that point rather than the grout line. This would mean half tile cuts on adjoining walls internal angles, than a half tile cut on bath end, then a second slightly larger cut to bath screen channel. I doubt tile for tile, you will have much more wastage than having the 60mm-100mm cut at shower screen.
Just a little time cutting.
But then again, I would then prefer the grout line on bath end to be central, so there then would be more wastage, as you will require two cut tiles above the 300mm, so they wont come out of one tile.