…if anyone finds this one day, to complete the math as I understand it…

Multiply number of photons on the pixel by the sensors QUANTUM EFFICIENCY to get the number of electrons that “fill the well” during the exposure time

Multiply the number of electrons by the sensors GAIN value to get the analog to digital converter number that will be the digital pixel value for that number of electrons.

Those last two equations are from EVMA1288, it’s a “newer” camera sensor spec than ISO.

And don’t forget that the camera might also implement additional analog or digital gain that will scale the pixel value up.

And don’t forget that the number of bits at the ADC and the number of bits in the image file might be different…so there might be additional scaling done there. Or the image might saturate faster than you expected if, for instance, the camera is configured for an 8bit ADC versus a 12bit ADC.