You don't need a tape measure to take rough measurements -- use your hammer, instead. Measure exactly 12 in. from the head of the hammer and mark the handle with a piece of colored electrical tape. Place another piece of tape 6 or 8 in. from the head. If you have a long-handled framing hammer, you'll be able to add a tape stripe at 16 in.
This time saving tip is especially effective when doing repetitive nailing, such as driving evenly spaced nails along the edge of a plywood sheet.