Hi Nick, The page in the manual that covers how the different layout options work is here. Steinberg manuals generally don’t do “problem solving”, i.e. taking something not-ideal that can occur and explaining it, but I’ll make a note to review whether any additional explanations in regards to casting off would be helpful.
In your case, I would recommend setting the defaults for rehearsal marks such that they are as well-positioned as possible in the parts, and then move them in just the score (rather than the other way around). Unless you want rehearsal marks that far away from the staff in the parts, in which case fiddling with your vertical spacing settings or even perhaps the casting off (such that fewer systems are on each page) could be worthwhile. Manually moving staves is very rarely necessary.
A similar question came up in a thread earlier today that relates to the circular calculation Dorico does when determining casting off, which might be worth checking.