Fix illuminance formulas to actually reach 0

[ThinkChaos]

Proposed change

See @javicalle's comment here #1599 (comment)

Additional information

CI failure is unrelated, tests pass if I rebase the branch on 0.0.117.

Checklist

• The changes are tested and work correctly
• pre-commit checks pass / the code has been formatted using Black
• Tests have been added to verify that the new code works

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.18%. Comparing base (915ff9b) to head (0a2d16e).
Report is 20 commits behind head on dev.

Files with missing lines	Patch %	Lines
zhaquirks/terncy/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #3297   +/-   ##
=======================================
  Coverage   88.18%   88.18%           
=======================================
  Files         301      301           
  Lines        9412     9412           
=======================================
  Hits         8300     8300           
  Misses       1112     1112           

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