Most people that list the angle of their drops are using elementary, but you can get a rough estimate of the drop angle using the grid in the NL editor and some simple trig, if you know how to do that.
trig is the answer. If you split up the drop segment so that there is a vertex at the maximum angle of descent, you can then double-click the vertex, revealing the coordinates of the handles, and then use trig to figure out the angle.
Select the steepest angle and save it as an element. Open it up in Purg and check out the slp-O: ? in the 'data' section. Thats what I always do and it seems to be very accurate.