The underside of your bath should be at around 21" or 500mm. So a full tile off the bath would leave a 140-150mm cut to the floor. Bit small in my opinion.
Looks like a full tile from the bath will leave a similar size cut to the top of the boxing.
So, roughly (and only based on approximations after viewing the pictures) a full tile from the top of the box would leave 150mm-ish to the bath and somewhere around 280-300mm to the floor.
Hazarding a guess this leaves around......100mm to the ceiling, no good!
Best place for that 100mm is on TOP of your box. This is where it cuts into your window too? I take it the boxing has been built to ledge height?
Effectively, your best layout for a 400mm tile laid portrait is a full tile off the floor. You might want to consider your flooring options first.
This is all based on a
standard room height of 8' (2400mm) or marginally under.
You should have around 250mm above your window so top corners of your window should be nice cuts too.
Hope that helps. Sorry if a bit speratic. Working it out in my head as i type. Just got in from work and i need my bed!