Blank space at Head of Page

I can't say why that particular default was chosen, but ai can say that for "most" scores, it is pretty reasonable. You score is unusual in that there is room for only one system on the page, but it doesn't come close to fillong the page. This is not the usual situation. Most scores would either have a several systems per page (eg, a piano score with around five two-staff systems, or a part for a different instrument with maybe 10 one-staff systems, or maybe a score for string quartet with two 4-staff systems) or else they would have have only one system but it fills the page (eg, a score for concert band). In the latter case, the page is already full, so there is nothing for MuseScore to do with that setting, so it doesn't matter. In the former cases - sveral systems per page - it usually looks bad for all the extra space to always be at the bottom of the page. Instead, the normal way music is published is to spread the systems out to fill the pae - the last system flushwith the bottom margin, and the rest of the systems equally spaced above. This works just fine in all the cases I just mentioned, and 70% ends up being a reaosnable value.

2.0 will do things a bit differently, so the fill algorithm won't always push the last system all the way to the bottom of the page but will allow you to set a minimum and maximum system distance. It sounded stange to me at first, but having used it some now, it's actually a great solution.