Measuring a Room by Yourself
Use an electrical outlet as a helper.
A good place to start measuring a room is at an electrical outlet. There are usually a couple of them on each wall, about 3 ft. from the corners. Stick the tip of a knife into the top edge of the outlet cover and the wall, creating a gap just big enough to insert the blade of the measuring tape. Carefully pull the tape to the opposite wall for your measurement. If you want the measurement to be accurate to within 1⁄8 in. or less, use a combination square against the wall, measure to it, and add the 1-in. width of its blade to calculate the overall measurement.
— March Coover, Dallas