Get a Peak 10X grain focuser with long mirror option and you can check grain right into the corners. A level to make the neg stage parallel to the base and then level out the lens stage.

Check grain left/right and front/rear with the Peak and if not sharply defined, change focus up and down to detirmine where shims or adjustments need to be made.

If you get sharp grain with lesn wide open, that is all that is necessary unless you are projecting a trapezoid which means the neg stage is not parrallel to the base.

Now you get a focus magnifier to use on every print. This is all I have ever used and I can adjust to get sharp grain corner to corner.