Post Deflicker simply writes some exposure metadata to postprocess the image. When you say "the settings weren't changing until the overall luminance/brightness (?) of the exposure got down to 10%", you are probably referring to the auto exposure method you have used (Canon meter, AutoETTR module or AutoExpo module).
For the existing footage, you have those options:
- You could write the script that changes all exposure values from the sidecar files by a constant amount (the hard way)
- You can run a different tool that re-computes the exposure compensation data from scratch (easier):
- if you use Adobe software, try
this script - for an open-source workflow, try the Deflicker module from darktable, which was actually ported from ML.
For ETTR, you probably want to increase the SNR limits, or allow more clipped highlights. Without seeing the actual images, that's my best guess.