if you're scene framing, it's my understanding that you will have decided the details of the lock ahead of time depending upon what you've already decided about the purpose of the scene.
In scene-framing play you wuld set the DC when the scene is framed, as part of the broader GM task of modulatig pace and theme.but what if you don't know what that lock is going to be?
This is why scene-framing play needs a system that makes this job easy (eg HeroQuest revised pass/fail cycle; 4e DC-by-level chart; Marvel Heroic Doom Pool; etc).