The right angle is pretty much correct but you are not taking equal distances on both sides of it. If you take equal distances to both sides then yes but isn’t it obvious that when one side gets longer the adjacent angle diminishes and viceversa?

None of this is obvious to me. I’m sorry, but you are talking to someone who is not only math impaired, but working with someone who is in worse shape than me, but fancies himself a semi-math dude.

Using those numbers, you have a triangle with angles approximately 90 degrees, 53 degrees, and 37 degrees. Those are approximate, as a perfect right triangle with a width of 40 and height of 52.25 units should have a hypotenuse of 65.8.

Just put in one or two new vertical 2 by 4’s so that the area you are trying to “triangulate” is now actually a perfect square. Then the diagonal member becomes easy to cut and measure.