How to get a 45 degree angle for focus tests

This is a potentially stupid question, how do I get a perfect 45 degree angle to perform focus testing? All the charts say you should angle the camera at 45 degrees, I dont know an easy and effective way to do this. I have tried using a number of methods, but my charts always look off centre, please help!

Use a level, most have a bubble at 45 degrees.
Or you could use Pythagoras Theorem.

If you are two feet away, you need to be two feet above the plane of the shot. As long as both distances are equal, it's 45 degrees.

The alternative suggestion is, don't do this. 45 degree focus tests are notoriously inaccurate. You can never be entirely sure where the camera is focused.

A proper test is to shoot perpendicular to a flat target. This leaves no room for doubt as where the plane of focus should be. Place a ruler next to the target to judge the placement of the DOF. Don't put the focus point on the ruler; it should be solely on the flat target.