I would basically place one of two conditions.
1) Ethnic Cleansing or Genocide. If that happens, clearly a secession should be possible, and actively supported by the international community.
Or some other huge discrimination.... the only one of which that's really relevant currently being the whole Palestine situation.
2) Lack of a National homeland.
So say, Scottish, Native Americans, Checyians, Baque, Kurds and Ugyurs.
Not Texans, Russian Crimeans, Albanians, etc.
So basically to allow a breakaway a country needs to either lose the right of the area through crimes or a group of people have to lack a country where they have self determiniation.
The main issue with just allowing any at all times succession is you have to decide what level of burden the leaving county takes... and it HIGHLY discourages helping other provinces and areas.
If I lived in New York, why would I agree to fund Georgian welfare federally knowing that they could leave at the drop of the hat.
