I agree that it has to be more than a simple majority of votes cast, but I think the voting threshold should be even higher than you suggest (maybe 60% by population)?  It is so fundamental a change,  so expensive and something that is almost impossible to undo.  The experience of independence could be great, but could turn out to be awful. There is no guarantee that Catalan will be let into the EU, if they want that, or be allowed access to the Schengen  zone.  They may have to negotiate trade treaties with the rest of the world from a position of weakness.  These things could all give a serious case of regret to the populace.  Given that downside risk, I think you need a solid super-majority to show that people truly want it before it can happen, because there just seems to be practically no going back.  The spurned rump of Spain is going to be in no mood, for likely decades, if ever, to let a remorseful separatist Catalan back in if things don't work out for them.