There are various ways to check and set the rotor blade angle. One method uses a flybar rod (which resembles the small auxiliary rotor blade under the main rotor) set horizontal with the help of a spirit level, a protractor fixed to the rotor blade, and a plumb line. The method does work, but the rotor axis must be kept as vertical as possible and the blades as horizontal as possible to obtain an accurate measurement. As so often, a little electronics can make life a lot simpler.