What Andy is saying is in effect, start a full tile either side of the window, remembering to allow for the reveal tile also, leaving a space in between.
Then whatever measurement you have between these two tiles, will be the size of your closing cut, remembering to allow for a joint either side of course.
Whilst in theory it's a good 'get out of jail free card' it's easier said than done. Some tiles can be extremely difficult to get an edge acceptable enough to have on show in the middle of the wall and a trial piece should be attempted first.
And obviously, you don't actually leave the space in the middle, you tile as you go, that was just a way of explaining it.