12' X 21' with 18" tiles?
Answers:
Lay out the tiles on the floor with the spacers, just don't use any thin set yet. You can start against one wall. See how much space you have left over. If the space is only a few inches wide, slide the tiles over so that you have a cut tile on each side. It is more cutting but it will give you a better look as the cut tiles will be closer to full size. Once you figure that out, make some marks on the floor and snap a chalk line both directions to give you a reference mark. When putting down the thin set, don't cover up the chalk lines. Put your thin set right up to the line and put down the tiles. Once that line is set, go to the other side of the line. The layout will take a while, but doing it right is work the effort.
Other answers:
use the least visible wall for the cut tiles. Pick a wall that has the most furniture or least amount of light. You can also start at the center with full tiles, and cut all the border tiles. Lots of work. Also, consider aligning this with any other tiles already in the home. In case you choose to finish out the home later.
use the least visible wall for the cut tiles. Pick a wall that has the most furniture or least amount of light. You can also start at the center with full tiles, and cut all the border tiles. Lots of work. Also, consider aligning this with any other tiles already in the home. In case you choose to finish out the home later.