If 99% of the packages in your repos are bit-for-bit reproducible, you can consider your distro to have a high degree of reproducibility.

And with even a little bit of imagination, it's easy to think of other possible measures of degrees of reproducibility, e.g.:

  • % of deployed systems which consist only of reproducibly built packages
  • % of commonly downloaded disk images (install media, live media, VM images, etc.) consist only of reproducibly built packages
  • total # of reproducibly built packages available
  • comparative measures of what NixOS is doing right like: of packages that are reproducibly built in some distros but not others, how many are built reproducibly in NixOS
  • binary bootstrap size (smaller is better, obviously)

It's really not difficult to think of meaningful ways that reproducibility of different distros might be compared, even quantitatively.