Same. Tell me if this makes sense:
In "target mode" there are two basic conditions to handle:
(1) a large cache write
(2) many small or medium cache writes
Case (1) has the potential to fill the drive if a high target (say 75%) is specified, so mover must run frequently enough to detect it and free cache space.
With frequent mover runs, assuming the age logic is granular (find sorted by age and iterate the files summing size), case (2) will cause very frequent array writes as a handful of files are moved on each run to maintain the target %. Better then to move the old files off in larger chunks, time rounded to the day (e.g. first all files older than 30 days from current date at 12am, then all older than 29, etc.)
That was all preface (sorry!) to answer your question:
I think it's almost entirely a UI decision. You could handle a target % and complex rules in code by calling your current rules logic in a loop from e.g. i=30 down, find older than i days until target free space is reached. And if it fails to free enough space that's on the user.
Your "Or..." which is my preference (your range is good), would be similar but skip over the complex rules logic avoiding failures due to user error.
Both proposals are a little hack-y (maybe you have a better idea) but I think until smart caching's built in every solution will be somewhat hack-y.
I really appreciate this exchange but please don't let me derail what's obviously a popular plugin to handle my (potentially minority) use case.