First things first, you'll need to grab an object (in my case, a ruler) that is fairly wide, lightweight, sturdy, and has a consistent thickness. It needs to be able to be placed on the bed, touching each Z rail, without any flexing of the object itself. Said object should also be at least one inch (2.5 cm) tall and at least four inches (10 cm) wide, though bigger still works. Other than a ruler, you can use a roll of blue tape, a small book, some cardboard boxes may work also. The goal is a consistent metric that will stay in place.