I'd try to work out from a centre line as well. I'd mock this up in illustrator first. The section around the door has potential to cause you real grief as it's almostbutreallynotevenclose to 2 tiles and the last thing you want is to start at the back wall and end up with a bunch of tiny slivers at the threshold - which is arguably the bit you'll actually look at the most.
I'd try to work out from a centre line as well. I'd mock this up in illustrator first. The section around the door has potential to cause you real grief as it's almostbutreallynotevenclose to 2 tiles and the last thing you want is to start at the back wall and end up with a bunch of tiny slivers at the threshold - which is arguably the bit you'll actually look at the most.