I'm assuming that you plan to place the decking at a 45 degree diagonal. Measure across the diagonal and check to see what you'll end up with before you start. I would start the decking in the center and work towards the corners anyway. You'll want to check the framing for square and if you need to tweak it a little the first long deck board will keep it in place. You'll want that board to be one that reaches both points of the diagonal. If the deck is rectangular, you'll have to find the center by squaring off of the ledger and the header from both corners. It may be easiest to square up the deck and fasten a temporary deck board to hold everything solid. If you start on the proper centerline, you should end up the same size block on each of the opposing corners. You can adjust the size of the corner block by shifting the starting position of the first deck board. Starting with it's edge on the centerline or it's center on centerline will adjust the size of the piece in the corner.