Hi Stan,

This is the page you're talking about
http://www.realcolorwheel.com/colorwheel.htm

Interactive Real Color Wheel Matching Tube Pigment Colors in RGB and YMC.

I think a spectroscope of dried pigment color would show all colors to be unpure.
The percentage CMYK was from mathamatical printed chart and matched the tube color which matched RGB equivalent matched by eye.

I matched the dried chip color to the computer by eye. It's close enough to paint with a computer tablet and compare the relationships of similar colors in pigments. 

I matched the pigment tube color dry by eye to RGB color and read it with a Digital Color Meter.