Zero
Further investigation yields a clue: If I change the Marker Type setting to 'gutter' I get this result:  which seems to show that the values have been transposed -...
If I then add two blank lines to the head of my config it corrects itself! I'll stick with this workaround for now, but it's some sort of bug?
Further note: having saved the file and re-opened it, I tried to duplicate the effect - unsuccessfully. So it's sporadic...
It's a mystery then: an inconvenience at worst...
It continues to happen here - but is intermittent.
Exact same occurrence on my Arch Linux box.