Example for the scanline debug mode on a Kobo Touch - bought it on Ebay, broken substrate, lower third of the display still operational. No idea what language it's set to.
With offset=105 the scanline is a tad too high:
It might work but it's not optimal.
With offset=75 it's too low:
With offset=95 it's okay:
The scanline should cross the letters in as many places as possible.
Note I had to reboot the device between each offset change
that should not be necessary, I'm still looking for that bug.