When we close or flag a question as off-topic, we have an option called
belongs to meta.physics.SE. We're also receiving questions that belong to Math.SE or Electronics.SE most of the time. So, it'd be easy for migration if there is a choice for
belongs to Math.SE and
belongs to Electronics.SE. Well, other sites do have that option.
For instance, we can take Android.SE. It's having a choice for
belongs to SO. I think superuser is also having such an option.