When I did my outside passage I marked the centre line at either end. Measured out at intervals from this centre line to make sure it was reasonably equidistant from the sides all the way along. His marked my longitudinal centre. Did the same in the other plane to mark the latitudinal centre plonked a tile right smack in the middle so that the corners of the tile intersected each line and worked out from there. I was using 6inch quarries. Mind you it scuppered me a bit when I found that the black ones were about 10mm smaller than the reds. I decided to lay all the reds then lay the blacks in the gaps.