I see a post on site A that I want to migrate to site B (where site B is not on the list of eligible sites for migration when closing a question as off topic, or I don't have enough reputation to vote-to-close yet).
What are guidelines should I follow before flagging that post for migration?
There are some guidelines we want you to follow when flagging a post for Migration:
It needs to be off topic for the source site, and it must be on topic for the target site. If a question is on topic for the source site, then it generally should not be moved.
It must be a high quality post. If you would vote to close the post on the source site (with the exception of off topic), then it more than likely be closed for the same reason on the destination site. We don't migrate crap.
The post should be recent. Depending on the destination, migrating the post could have a number of negative impacts such as the OP not being registered on the destination site, the OP losing rep on the source site and more. Note that in the case where the question has no answers, this does not always apply.
The post should not have an accepted answer or a lot of answers. Either one is usually an indication that the problem's been solved; so migrating it won't really allow for new or better answers.
Please keep these guidelines in mind when flagging posts for migration. If too many of your flags are declined, your flag weight will drop and your flags will become meaningless.