Using Perry's diagram for the neck angle, trigonometry can be used to calculate it. When dealing with a right triangle, the tangent of an angle is equal to the length of the opposite side divided by the length of the adjacent side. In this case, the right triangle is formed from the bridge, the bottom of the fretboard where the body joins, and the guitar top under the bridge. The right angle is at the bridge, and the angle you want to find is at the fretboard. The legs of the triangle are the bridge height and the length from bridge to FB, and the hypotenuse is the length from the FB to the top of the body under the bridge. Tangent = Opposite divided by Adjacent. Therefore...
Tangent of the angle = bridge height divided by length from bridge to fretboard. Let's say your bridge is .5 inches high, and the distance from your FB to your bridge is 6 inches.
Tan (x) = .5 / 6
Tan (x) = .0833
x = arcTan .0833
x = 4.762
The angle in this example is 4.762 degrees.
Pick whichever method works best for you. Both work fine.