When sighting in off the bench, move your head back to where you are only seeing half of the image through the scope.
That will eliminate most, if not all of, the potential parallax issues.
I sight in all of my guns for a 200 zero, with zero regards to the scope's parallax setting.
Proper scope mounting for eye relief & height should be where you can close your eyes, bring your gun to bear, open your eyes, and you should be looking through the middle of the scope w/o needing to move you head to do so.
Shooting out to 400 is really a non event in worrying about fixed parallax in a hunting environment.
I've shot a five shot 12" group at 1400 yards with a 10X & fixed parallax, you just need to be looking through the middle.
