I agree with the others that you're close enough as is and you're doing the right thing in taking all the measurements with the meter facing the same direction so that the readings are consistent. If you want to double check the incidence and/or calibrate your Robart meter for future reference, measure off your bench top to the center of the leading edge and to the trailing edge (the difference being side a), and the cord of the wing where you took the measurements (side c), or the horizontal distance between the leading edge and trailing edge measurements (side b), and plug them into one of the many Right Triangle calculators available on-line.