With the Colrona shader you miss the influence of all the root values: Reflection, refraction, self illumination ecc.
The Iray Material for example when it gives Reflectance and Transmittance values takes in count all of the values in the material and I think this is very useful and great!
In any case here is how it works (very simple):
To be physically corrected your Reflectance value must never go over 80% / 85%. A white paper, which is the most white element we have in the real world, after the snow, should use 80% of Avg. value. A white paint wall is around 65% / 70% and so on...
What is the Reflectance value? Very simple: A paper (Avg. 80%) receives 100% of luminosity and reflects 80% of it in the space.
The Avg. value represents the average value of your bitmap while the Max value reads the most bright pixel of it. This is very useful as you may have a 65% Avg. value while some pixels are positioned to a 90% Max. and this is not ok. In this case you should go in Photoshop and lower the bright levels of it till you read a lower Max. value.
It's a shame we can't use this tool in Corona as it can help a lot to avoid easily albedo problems.