The best practice use for Downgrade protection is of course "full protection" which means from a customizing setting point of view, an empty table, meaning "no exceptions" for DGP calculation.
You can reach these setting via IMG:
Select "Configure Downgrade Protection" and you will see the "Downgrade Conflict Type" first.
Here we do not want to do any changes and normally this is also where you never change. It's the table to configure what kind of warning or error should appear with which corresponding icon. I guess the standard setting to have the release check being a warning, the re-assign check warning too, the predecessor check critical and finally the imminent check very critical make perfect sense and therefore i never changed this.
We want to browse to the next setting which is the "Downgrade System Configuration".
as said before in standard this table is empty which means no exceptions setup, which results in full protection and a DGP conflict calculation for every conflict type (Release, re-assign, predecessor, imminent) for every system, client, system role and project.
Now i case of a dual landscape we have retrofit setup to look for conflicts between the 2 landscapes and calculate the corresponding retrofit type.
In case of no conflicts -> green case in case of conflicts -> yellow or red.
As the retrofit conflict calculation happens at the time of when a Transport is released in the retrofit source at the same time (or to say it technically correct before the retrofit calculation takes place) the DGP check will be performed.
As a consequence DGP will issue a conflict (of type warning) before the transport can be released and before the retrofit can be calculated.
So the DGP conflict has to be ignored first before the transport can be released. So we can more or less say for every conflict that will be handled via retrofit with a yellow or red type (conflict case) the DGP check will stop the process before.
This is of course an unwanted behavior, as the transport has to be released anyway and the conflict will be resolved using the retrofit tool.
Therefore i recommend switching off the release check of DGP for the source system in dual track retrofit landscape.
You can exclude that check from being performed by adding a line into the "Downgrade System Configuration".
Like that, when releasing a transport in the retrofit source system there will be no DGP check for that transport. The transport will be released and the retrofit calculation will take place and will put a yellow or red case. So you can solve the conflict by using the retrofit tool and will not be stopped by DGP every time before.
So i would resume that in every dual retrofit landscape the DGP customizing should have that exception that the DGP release check is switched off for the retrofit source.