The only way you can accurately test focus on an SLR is with an auto-collimator. Eyeballs won't work.
The auto-collimator will measure any error that exists between the optical path from subject to film and from subject to GG.
The split-image rangefinder is coplanar with the GG surface by design and manufacturing process. It's correct by definition.
If your eyes do not render the resulting GG image sharply, you need to add a diopter.