you might want to read up on the proper way to lay out new flooring. you want the floor job to look symetrical and even on each side. this is done by finding center and working from the center out in four ways.
Anyway, your question about mastic. mastic must set for about 15 minutes before you apply the tile. use the proper trowel size for your quantity of mastic. If you put down the tile on wet mastic, it will continually ooze up from the floor in the cracks or edges of tile. Read the instructions on the mastic can, it will tell you the program.