I sincerely apologise if this has been asked before.
I am doing layout on a score, two systems per page, no justification horizontally. To achieve this, I am now putting in system or frame breaks, one at a time, and checking how the break affects the final page of the flow. It works quite well. However, what I thought about was to start a flow with a system break and in the properties in engrave mode under Format being able to set Note spacing here, just like it is possible to set Space size. I think this could be useful. I am not competely familiar with how spacing is calculated in Dorico, so please correct me if I am wrong.
Edit: The Casting off features could also work in this way, maybe?