Rod N, there is a lot of good advice on this forum regarding porcelain tile if you do some searching and it may make you aware of other things you may not have considered.
As far as laying it out, I wouldn't recommend starting in the corner like that. Rooms are rarely square and as you build out from there it can start causing all sorts of problems.
It's generally best to start your first row all the way across the front at the garage threshold and build out from there. That way any discrepancies in the room not being square are taken care of at the sides where you will cut the tile to fit. It also helps tremendously to lay a few chalk lines to keep things running straight.
It's usually best to measure it out side to side not forgetting to include your grout width before you start. The tiles that are cut to fit on either end look much better when cut to fit in larger pieces. By doing this you don't run the risk of one side of the garage not running true when trying to use a full tile without cutting.
This will also allow you to find the center tile row in order to lay a chalk line to help keep things running straight.
If you haven't found it already,
read Dakota's thread here on his garage tile build. He does tile as a living and gives good pointers. I believe he did his garage one half at a time.
I'm sure there will be others glad to help you out as well.