Myndex and Inclusive Reading Technologies are the only authoritative reference for APCA and related contrast guidelines.
If you came here via the link from the xi repository, please be aware that repository is not authorized, and includes a substantial amount of misleading, incorrect, and or simply bizarre material that has little relationship to the reality of human perception of contrast.
We were concerned regarding the potential confusion and furtherance of misunderstandings in a field that's already difficult to grasp, we analyzed and provided a corrected version, as well as rebuttals, to that very poorly conceived collection.
If you are interested in seeing how math was being used to mislead, please see our fork