Of course there is, and anybody who has used a part of the propeller as the CG datum is a dangerous idiot who needs shooting, slowly - because an engine change, or slight change to the engine mount geometry will muck up everything.
I'm guessing that all of your drifting CG ranges and limits are down to various people re-inventing the wheel over the life of the aeroplane, and this can potentially be quite dangerous.
Many manuals do use %MAC, usually with conversions from inches/metres/centimetres from a fixed datum - wing leading edge at the root is usually most reliable, although mainwheel axle centreline works quite well to simplify the maths, or just a fixed bulkhead somewhere on the airframe. The BAe Hawk simply has a small plate under the belly with a line on it engraved "CG datum" - an elegant fix !
G