If the parking doesn't have a docking system, you might enable the Marshaller distance readout in the GSX - Settings ( should be enabled by default ), and that will help a lot with parking.
If the parking DOES have a docking system, and it's an FSDT scenery, TRUST the docking system, because the evaluation obviously interact with it.
If the parking DOES have a docking system, but it's not FSDT, so you are using a 3rd party scenery with its own docking system, then it's best you just forget about the parking evaluation, unless that 3rd party developer did it so well, than the docking system is perfectly tuned to let you always park with the airplane reference point exactly in the middle of the AFCAD parking spot, *regardless* of the airplane type, which is not very easy to do.
We do it, of course, in all our Docking systems, but can't guaranteed that every 3rd party developer would be able to do that.