The best way to add such an option, as I see it, is make it combine with the per-file options.
For example, if you set global up 10%, and file down 10%, then you get 0% shift.
All other ideas I can come up with to manage the options and allow per-file overrides to be sensible result in unexpected outcomes.
The remaining question is, what do I do with files that already have an adjustment? Do I keep theirs as absolute and transition to a relative adjustment when they are touched the first time? That seems like a sensible thing to do.