I think the angle you are measuring is the opposite angle as the one you are showing is bigger than 90. So my guess that your A is 95.8. What I show will give you the 5.8 part

To calculate the angle is complicated because the back is generally sloped relative to the heal and tail block. you can calculate the angle first assuming there was no slope; the arc sitting on two blocks the same height. But you really need to include more information because it is an arc and there are an infinite number of tangents.

What I would do is to use the calculator at

http://www.liutaiomottola.com/formulae/sag.htm to calculate the sagitta of the arc for your back. Using the same units (inches or feet) fill in half the length of your guitar and the 15' radius and get a saggitta.

Then go to the bottom calculator in

http://www.liutaiomottola.com/formulae/sag.htm Calculating Height of an Arc at Any Point, and calculate the rise at the end of your heel block. For X you would use half the length of your back minus the thickness of the block.

You can use this height to get an approximate angle of the block by calculating the tangent using the height you calculated above and the thickness of the heel block. The tangent of an angle is tan(θ) = Opposite / Adjacent of the sides of a right triangle. In this case opposite is the height you calculated just above and the adjacent is the thickness of the block. Divide the opposite by the adjacent and use an arctan calculator for the angle, just make sure you are getting degrees not radians.

http://rapidtables.com/calc/math/Arctan_Calculator.htm Now you have an angle representing just the contribution of the sanded radius that you can add or subtract to the angle of your block that is based on the slope of the back.