There is not really much to say. Ideas?
We don't need to. It's perfectly OK for sites to have an overlap. Migrations should only be done if:
- The question is off topic on the source site
- If not, it still may be migrated on OPs request
- If it is borderline on topic on the source site and doesn't get answers, again it can be migrated.
No need to draw a line. It's fine if more than one site gets a piece of the pie.
But clearly there questions which do exist which are far better suited for a physics stack exchange site. If people ask questions regarding circuit diagrams on the astronomy stack exchange site, that would be bad. There are mutually exclusive regions, and the tricky part is determining what those are. Optics is clear example of this overlap - blocks on inclined planes are not.