Re: Proportionately scale other staff font sizes
Uh? I can't see why.
Of course, due to rounding, it can't be perfect
It is not documented, so one cannot be sure what it does or is supposed to do. Due to rounding, it is not useful. 99.44% of boxes running NWC have a floating point processor. IMO, NWC should use it more 
I used it many times to "fit" the score on the page.
Unless you have lyrics, or a staff font with 'Preserve Width' set, what "fits on the page" is unaffected.
Of course, I wrote about "user 1", but all the user fonts are not affected.
Why?
Because it is useless.
Anyway it seems I'm the only one using the automatic scaling.
Doubtful. It is checked by default. I leave it checked, but more often than not, I need to make additional adjustments to fonts when I change the size of the system font.
Nobody seems to care about it.
I care. I'd like to see it documented, apply to both staff and user fonts, and for those fonts to be specified as a percentage of the system font, rather than expressed in points.
Windows also allows "half points", i.e., 8.5 points. You can try it in WordPad. There may be technical reasons why NoteWorthy only supports even sizes for the system font and integer sizes for the others, but if not, it would be an improvement to support additional granularity.