You should see the wands crossed but, if you see the marshaller making one step ahead with one foot, and then starts waving the wands back and forth, it means you went too far.
And, keep in mind it's not just the back/forward position that gets evaluated, but also the angle of approach, even if you park perfectly at the correct distance, if the airplane is a bit angled to the yellow line, the parking will not be considered good enough.
Note that on some 3rd party sceneries, it's not guaranteed the yellow line matches *exactly* the underlying AFCAD apron path, and GSX knows only about the AFCAD path so, while approaching, trust ONLY the marshaller for left/right alignment, rather then the visual scenery. Default sceneries are sometimes easier to use, because the AFCAD IS the visual scenery in all cases, while on 3rd party sceneries with custom ground, it might not be.