50% is probably unrealistic. Nobody really wants to diminish their storage by 50%.

Let's set a fixed threshold -- 100GB, say -- and play out both methods.

Method A: One or more ballast files are created, totalling 100GB. The machine runs out of storage and grinds to a halt. Hopefully someone notices soon or gets a generic alert that it has ceased, remembers that there's ballast files, and deletes one or more of them. They then poke it with a stick and get it going again, and set forth to resolve whatever was causing the no-storage condition (adding disk, cleaning trash, or whatever).

Method B: A specific alert that triggers with <100GB of free space. Someone sees this alert, understands what it means (because it is descriptive instead of generic), and logs in to resolve the low-storage condition (however that is done -- same as Method A). There is no stick-poking.

Method C: The control. We do nothing, and run out of space. Panic ensues. Articles are written.

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Both A and B methods have an equal number of alerts for each low-disk condition (<100GB). Both methods work, in that they can form the impetus to free up some space.

But Method A relies on a system to crash, while Method B does not rely upon a crash at all.

I think that the lack of crash makes Method B rather superior all on its own.

(Method C sucks.)

A + B would be best. Warn at 200, file to reserve the last 100 (or 50 or whatever). That way if the fill is too fast to react to in time, you still have a quick way to temporarily gain disk space, if needed to solve the problem.