In one of my previous lives I worked as an engineer installing weapons systems foundations on Navy ships. This and other duties required very precise measuring techniques. I’ve seen many tips for marking accurate lines for cutting when using a square, and there are many very accurate squares available. What the users might not understand is that the shorter the measuring tool, the easier it is to inadvertently introduce large angular errors. This can be demonstrated by putting a penny under the end of a 3 foot straight edge, and under the end of a 6 inch straight edge. The angle introduced is much larger on the shorter straight edge. (To the editors: It might be good to make an illustration here.) So, my tip is: Use the LARGEST square that you have that will conveniently fit your measurement. Holding the short arm of a square on the reference side, and marking with the long arm is how we all work. A 6″ square will do to make a cut on a 6″ board, but when a high degree of accuracy is paramount, use a 12″ or even an 18″ square.
– Mark Messer