The basis for secession cannot simply be a recent dispute over policies. Disputes over policies happen everywhere every year. It has to be based in a combination of:

1) Joint unique identity of the people seceding when compared to the national identity

2) Logical geographic location (meaning can you draw a good border, and if you did would it hurt others within that border who don't want to secede?)

3) Extended precedent of the nation as a whole exploiting or harming or ignoring the needs and wants of the group asking to secede.

4) Irreconcilable differences of values and beliefs.

I'm not sure if Catalan meets all 4 of these (but I bet it does). Certainly the Kurds, whom have been carved up and suppressed by multiple bordering countries for generations would qualify. In fact I'd say another unofficial criteria might be if the group is organized enough to have it's own militia big enough and strong enough to take on most of their rivals then they're pretty much successfully acting as a separate state already. Just let them be what they're asking to be.

I have but two considerations that I would take into account. First and most importantly, the majority of the people in the region must want to secede. Secondly, as you have stated, it must be possible to draw a border that only includes the region desiring to secede. All else is irrelevant if we believe in a system that requires the consent of the governed.