- Using your drywall square, place the long end of the square's "T" perpendicular to the floor.
- Hold the longest part of the square roughly parallel to the floor.
- Mark a line across 4 studs by running your pencil along the bottom part of the straight edge.
- Drill holes at this height.
Since NEC does not specify how high to mount receptacles or drill the holes, it's all a matter of preference.
Is this persnickety? Can you just wing it and not measure out your drill heights? You bet, and I've done this many times. But I like getting the height uniform for each stud, because any variance in height slightly increases the friction on the Romex® as I pull it through.