I have a full score where two systems can fit comfortably on some of the pages. Other times only a single system can fit. But automatically justifying makes the staves too spread out. So what is the best way to split the difference and make a single system only fill about 75% of a page? Should I adjust the frame itself on each page and then do the auto justify thing?
Probably what I would do is indeed to change the height of the frame on that page such that the music frame can be fully justified within the height of the frame.
I probably wouldn’t go to the hassle of creating a page template with a different frame height and then using a page template change to use it, because although that would avoid creating an override, if the casting off of the music changes and a different range of music appears on that page, I would need to make a decision about what to do in any case.
Making a custom page template with a shorter music frame is really mostly helpful in situations where quite a few of your parts have the same number of systems on e.g. the last page, and they will all need shortening a similar amount – having the shorter page template means you can just plonk that on the last page in as many layouts as needed.
But if you have a fair few pages in the score that need this consistently shorter music frame, that could be worth making a page template for – makes it easier for you to know for sure that all your single system pages have matching formatting, and even staff spacing.