I don’t believe that we perceive luminance in a linear fashion, but the systems of measurement aren’t straightforward coming at it as a layperson.

With sound, a 10 dB increase is 10 times more intense, but it doesn’t sound 10 times louder to the human ear - it sounds (roughly) twice as loud. So if something was 6 dB quieter (1/4th as energetic), it would sound maybe 2/3rds as loud.

The next things to ask are:

• does an obstruction of 80% of the sun result in reducing the light we receive to 20% of what we’d otherwise receive?
• how does a change in light energy affect our perception of brightness?