Yes, it's from NASA. Apparently they measure atmospheric optical depth (tau, where higher values mean more blockage).

During storms, the issues is less about dust settling, but about the blockage in the air.

Seems like around 0.6–0.7 tau is normal. Storms have reached over 10.

The most prominent example is the 2018 global dust storm (Martian Year 34):
It peaked with a tau of 10.8 at the Opportunity rover's location (Perseverance Valley).

• Tau values exceeded 9–11 in many regions.
• This corresponds to blocking over 99% of direct sunlight in heavily affected areas, effectively turning day into deep twilight and starving solar-powered missions of energy.
• The storm ultimately caused the permanent loss of the Opportunity rover in 2019, as its panels received negligible power for months.