There is no cut in stone rule the timing has got to be spot on 28 degrees. That is a good starting point for a lot of engines. Do you know that a lot of old time model airplane ignition engines had a lever to set timing which is advancing or retarding the timing. Get your engine huming (top rpm) and set timing where top (peak)rpm is achieved and if needed adjust carb for good low end idle. Also do not be dumb and run wide open very long. Drop the top RPM by setting the carb richer. Say maybe drop it about 100 RPM. If your engine runs good...do not be concerned about the 28 degrees. Capt,n