The 2 Graflex RH10's and the RH/8 here are all within about 0.197 +/- .003 inch
The newest one, a Singer with the blue handles, is close to parallel on all 4 pin ends,to the limit of my calipers within 0.001 inch ( 25 micrometre)
A 103 mm lens in 6 x7 at f/8 focussed close in (at 3 metre) can have a DOF of 350 mm which corresponds to +/- 0.25mm or +/- 0.010 inch of throw on the film plane.
Boring numbers, but they put the accuracy of parallel film in perspective. The Singer back prices range from $80 to $100.
The 2 x 3 Crown design actually is intended for the user to fine tune it using the front standard adjustments and the ground glass.
The rails need to be dropped to the second click.
For example-For mid to long distance work,
For side to side sharpness, set each rail stop so that infinity focus is even, that no side ever goes beyond infinity.
The lens can be tilted down slightly so when the tree-tops etc are at infinity, the foreground will always be on the sweet side.
For example 1 degree, or 1 mm (0.025 inch) of down tilt on the lens board might be a good setting to bring the lower foreground slightly into the DOF.
It is also possible to pre-set a small amount of front rise.
Then lock all the adjustments, and just use the camera with the rangefinder.
However the cameras won't fold up when set like that. I have an old video bag to carry them.
For close up work,(and the bellows can extend out out to about 160mm) it is probably best to set the camera case- by case.
The 2 x 3 Crown here is quite tight after the knurled screws are locked, except there is a little bit of flex in the lens tilt.