Holding a pencil up looks theatrical and is genuinely how it is done
The method
Pick one measurement, usually the head. Hold the pencil at full arm's length, thumb marking that length, and compare everything else to it.
Arm locked
The arm must be fully extended every time or the scale changes between measurements. This is the part people get wrong.
Angles
Hold the pencil along an edge you can see and bring it to the paper without rotating it. Angles are as commonly wrong as lengths and are easier to fix.
