1. The Munsell Saturation ruler is in 11 equal segments in steps of 10 L (0 to 100). The Zone System Ruler follows the system defined by Norman Koren (http://www.normankoren.com/zonesystem.html) which I believe is more in line with the idea of the Adam's Zone system, than equal segments, for digital images and has less segments than the original 0 - X.
2. The advice by Frank Reilly to painters on how to mix colours as the object became darker is generally accepted as correct and follows the old masters. Basically, as an object colour in a scene being painted becomes progressively darker, its Munsell value, not surprisingly, decreases. The rule of thumb says that the chroma also decreases, by about the same amount proportionally.
Although it seems counterintuitive, the hue of a shadow is not nearly as important as its chroma. As the shadow becomes darker, and the chroma decreases, the hue becomes nearly indistinguishable.
Paul Centore has written an article (http://www.99main.com/~centore/ShadowColoursForPaintersLowRes.pdf) that explains all this, but in a nut shell it says...
Suppose that the colour when lit has Munsell coordinates H V/C
For example 10GY 7/12. The left axis is a vertical line of neutral greys, numbered from N1 through N9. Continue the neutral axis downward, to where the value would be -1.
Draw a line from the value -1 on the neutral axis, to the center of the square containing 10GY 7/12.
The shadow colours of 10GY 7/12 fall on this line.
This is obviously approximate as the chances of a colour exactly matching a Munsell swatch is not that high. The Ruler swatch values use this formula, starting with the selected colour (as denoted by the '*' swatch).
I find it quite informative to use these rulers on images and especially to guess/check Zones when dealing with coloured objects - I find it hard enough with grey-scales, but colour is a whole different ball game!
My odd experiments seem to agree that the Reilly saturation model creates 'better' looking images than straight hue values, especially when we boost colour (for impact effect) compared to the original. Without a doubt simply using a L mask during Lab colour saturation helps our perception.