How much local storage is needed for iCloud Photo with “Optimize Mac Storage” on?

Photos on macOS has a preference under iCloud Photos called “Optimize Mac Storage” that results in the replacement of full-resolution photos/videos by thumbnails/low-resolution photos to preserve local storage.

I have created a separate 75 GB APFS Volume on my 1 TB disk to keep the photo library size under control. This has worked well generally, but lately I’ve gotten an error saying the library can’t sync because there’s not enough local storage.

I imagine there’s a minimum fraction of the total library size that’s needed locally to store thumbnails/low-resolution version of the photos, but in typical Apple fashion, this information is not available anywhere that I know of. So the question is:

How much local storage is needed for iCloud Photo with “Optimize Mac Storage” on?