The pics don't really help.. As CR says, don't come off the floor with a full tile. Only you can see if that will work with your other cuts, but chances are, neither the floor or ceiling will really be level (although i have seen a level floor once lol!). 😉
BTW you may want to think about tanking around the bath/shower area if you haven't already and lastly I see most of the boards are dot and dabbed but the shower wall looks like it's been nailed, i could be wrong and it's been screwed but if not again best to screw it not nail 😉
Make sure you check you won't have any small slithers around the bath or window before you start fixing. It's not as simple as deciding how big you want the cuts top and bottom if it leaves you with a 20mm cut round the bath!
Really? there looks like a lack of fittings in the centre of the boards, you didn't just screw round the edge of the boards did you? Or did the pictures just not pick up the screws?