1 - delete your old prefs file
2 - rename my old prefs file
3 - download your prefs file again
4 - rename it with XML
5 - move it into User Config
6 - open Inkscape
7 - type text size 30, line space 40 px
8 - type text size 72, line space 40 px
9 - resize page to contents
10 - take screenshot with About screen
11 - save SVG
Sorry, it still looks like the line spacing has to be set for every different font size.
(Odd that my prefs file is 3 times larger than yours. Maybe the answer is in my old prefs file?)
Edit
I did find it interesting that the line spacing was set to 1.0 with no units, when I opened 0.92.4 with your prefs file. I'll have to switch out the prefs file again, but I don't recall seeing any way to not have units for line spacing. Let's see....
Oh, look at that! There is an option for no units, even with my old prefs file. Is that the solution, just don't use any units for line spacing?
Hey! That works! Using 1.0 line spacing, and no units, does appear to create automatic line spacing! Awesome!
Not that I understand it any better. But I'm thrilled there's some way to do it!