I set the slope selector maximum to 0.8, and the dialog says that’s equal to 51 degrees. However, this is incorrect since a slope of 0.8 equals 39 degrees.
Or maybe the decimal value equals the sine of the angle? But this does not seem to be the case either.
Also, I think it would be great if we could input our own values instead of just using the very imprecise slider.
The value reported is actually a “normalized vertical scale independent angle”, the original idea was to make the slope selector not change when the world vertical scale was changed. However, this has proven to be both confusing and not usually what you want. Already in for the next dev build is a modification to the slope selector to work natively in degrees (and let you enter them numerically too). So… coming soon, more better things. 
Awesome, thanks!
Hi again posfan12 - this one is also in the latest dev release! Enjoy!