Measuring for floor tiles can be tricky...

The first layout consideration in floors is based on your personal vision. What will your eye first see? Sometimes this is at the point of entry into the room. If the room has multiple entrances, the eye may naturally travel to the longest wall. I would say that in the case of a bathroom, the eye would first go to the base of the shower or tub. Each room must be considered individually. Sometimes in layout there are many choices that will work. We need to pick the one we feel will "look" the best.

In the case of tiling one room, take measurements in both directions. Compare these measurements to the tile and joint size chosen. This will indicate whether full tiles, or cuts are necessary. If you end up with tiny cuts, less than 1/2 of a tile at one wall, it will be more visually appealing if you shift the field so that the larger fuller tile is used even if you have to cut it at both ends. 

Tony's Tip: When you identify your longest wall, make your first working line far enough from the wall for at least one trowel. This allows you to use the line and still apply a bond coat between the first tile set and the wall.To establish the line, measure out from the longest wall at two places and make reference marks. If the room is large, over 6-8 feet, use a chalk line. If not, you can use a straight edge and pencil. 

Tony's Tip: When using a chalk line, be sure that you carefully align the chalk line with the reference marks and pull the line as tight as possible prior to snapping it. Also, place the chalk line as close as possible to the reference marks, while aligning it, without actually touching the surface. When the surface is touched, chalk will be left. Always retract the line into the tool for each line snapped. Remember. the chalk line aint worth nada without chalk.