How to nail focus with Digital Split Image on FujiFilm X-Series cameras?

FujiFilm X-Series cameras offer Digital Split Image focus assist mode. The manual provides inadequate usage instruction, stating:

Digital Split Image: Displays a split image in the center of the frame. Frame the subject in the split-image area and rotate the focus ring until the four parts of the split image are correctly aligned.

How can I use the Digital Split Image focus assist mode to get sharp, in-focus images?

When I have tried aligning the four parts of the split image works, it okay for gross focusing, but appears inadequate for fine focusing. How can finer adjustments be made using this mode? How is it useful if only rough adjustments are possible?

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  This question is not about using the menu to change settings.

  
  
  


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  I expect use of this focus assist mode to be similar across X-Series cameras. If not, I am primarily interested in the X-T20 and X-H1. Secondarily interested in X-E2/S. Not interested in X-T3/T30.

  
  

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