If we were just using 'dome flats', I could easily come up with an algorithm for acceptance. However, sky flats are quite subjective. You might have a plane or meteor. A local light comes on or off. The moon rises or sets.
Twilight starts or ends.
The telescope field of view passes through M31 or the Milky Way, or Sirius. As long as you are trying to make flats from sky exposures, you have to manually inspect each and every one. The flat is the most critical image for the entire night. Any systematic problem effects every processed image.