In theory yes ($102 is the way to go), and while I would expect the leadscrew/ballscrew Z axes to not usually require much calibration if any, I never owned a Z-plus (or its SO Pro variant) so it’s not impossible. I am a little surprised by the 0.3% error in your test case, but can’t tell if that is typical.
Anyway there is no harm in modifying $102, I would maybe just do two more tests:
- checking whether there is any backlash on your Z. Depending on the way you measured it may or may not be included in the 50.63mm result. If the difference to 50.8mm happened to mainly to due backlash, there would be no point in adjusting the number of steps per mm.
- repeating your test for various shorter and larger travel distances. If the calibration factor you get is roughly the same in all cases, then it’s a good case for adjusting $102, if not then you may enter an endless tail-chasing loop where the perfect calibration for a small distance is off for the longer moves, and vice versa. I have spent a ridiculous amount of time obsessing about calibration (this, was probably the best example of me going way too far), I now run my machine with the default steps/mm on all axes and compensate (when needed) at CAM level.