Jazz Lead Sheet Question

Good call, that is indeed the relevant parameter. I'd just make a few additional observations:

The settings in the Jazz Lead Sheet template were chosen to allow you to find a good balance between using large enough sizes to sightread and wide enough spacing to allow for chord symbols and other markings on one hand, and yet also fitting a decent number of staves per page on the other hand. It's kind of a delicate balance. So do realize that increasing that margin value is likely to give you fewer staves on that first page, unless you make up the difference somewhere else.

Also, keep in mind that some of the settings in that dialog will not appear to take effect if your page exceeds the "fill threshold" specified toward the bottom of that same dialog. Any page that is more full than the specified value will automatically be stretched out to fill the page. I actually took advantage of that in the template, and as a result you'll see that score with more than five or six (?) systems will be stretched to fill the page, and thus be more widely spaced than a score with fewer staves. There was simply no other way I could find to get the spacing I wanted in all cases. So it is to be expected that you might need to tweak settins depending on how many systems are on your page, and similarly for the second page, which will come out differently because it lacks the title frame.

A useful trick for those short scores that have only a couple of systems and thus appear to right is to simply add a vertical spacer below the last system. Stretch that thing downward intil the page fill threshold is exceeded and the automatic stretch kicks in, and suddenly things are spaced more generously. I usually stretch that spacer all the way until it actually pushes the ladt system pnto the next page, then back up (using the cursor keys) until the last sustem just returns to the first page. This tends to produce nice spacing in a lot of cases.