Calculate Slope Angle of Doubled Sloped Shape (Offtopic Question)

I know this is off topic, but If someone can help that would be great

I have a shape that has double curvature/slope.

To calculate the slope of a triangle you can measure the angle between its Normal and a Z vector.

If you apply this procedure to 3D shape, lets say a surface. You draw the line normal to its barcycentre, and measure the angle. In this specific case the angle is 14.96 or 15.

Supposedly thats the average slope of the shape. Yet if you measure the angle of both slopes and do an average you get 11.505 way far from 15.

The green triangle represents the resulting base of a triangle with same height and angle 14.96.
The black triangle is simply the middle triangle. Its slope angle is 11.39

What is the slope of this shape? Is it 15? Or 11.505? or 11.39?

Im doubting with the 15 value, as you can see, the green triangle does not even reach the end of the original shape. But yet the angle between the normal and the Z vector is 15.