An idea for measureing grade would be to use a piece of string. Make a big loop and secure two points on the hill with some weight, make a triangle of the string and then it's a matter of geometry. If you use a plumb bob for the downhill side and line its line with the string "triangle", it should be easy to mark the anchor points and the angle on the downhill side. Once there, A^2 + B^2 = the length along the ground, then use trig to get the angle of the slope.