Hmmmm... all the tents I've used, I lay out with the floor on the ground, outside face down... then fold one edge into the middle, then the opposite end into the middle, then outer edge to opposite edge, and repeat if I have to, then roll tightly, squeezing out air as I go, to fit the bag that goes on the top of my rear rack.
This way, any dirt on the floor remains separated from the rest of the tent. It's a neat and tidy way to fold and easy to unfold, too. By using a separate rack-top bag (in all cases, the one the tent came in), I keep the pannier contents clean and dry, and if I need to dry the tent later in the day, I can unpack it and lay it out.