Usually you set the cd/m² value based on the brightness of the room, although 80cd/m² is no magic figure, anywhere between 80 - 110cd/m² is usually a good place to start on my opinion
Ian, î do realise you have done all sorts of work around this subject and I don't want to denigrate, in any way, your findings.
But I come at this from a mainly computer background rather than a darkroom approach, which might explain why I think differently.
I can't remember exactly where I stumbled across the 80cd/m² value but, so far, all I can say is "it just works"
So, I got my light meter out and measured the light falling on a sheet of baryta paper,
in incident mode, in the bright but cloudy daylight in our conservatory, where I evaluate prints. The result was 1/100sec @ f/3.0 100 ISO.
Then I took a spot reading
in reflective mode of an empty browser window on my screen at 80cd/m² . The result was 1/100sec @f/2.4 100 ISO.
So, about ½ stop difference - possibly less if the faint sun hadn't peeped out.
And this is how I evaluate my prints - in diffuse daylight without direct sun, because these are the conditions under which the prints will be usually be hung.
So, in my experience over many years, 80cd/m² works very well, especially when you consider that most (non-professional) exhibition spaces are not as well illuminated as my conservatory.
I would be very interested to see what you find by doing a similar comparison of reflective from your screen with incident from in front of some paper in a "normal" viewing light.
