Composite RGB Curve math

In Photoshop's Curves dialog (CS3), you can add control points to the composite RGB curve by command-clicking an area of the image. I've tried to figure out how the location of the added control point is determined from the color that was clicked on, without success.



For example, command-clicking on a solid of RGB (127,127,190) results in a control point at 134,134. I can't figure out the relationship between the source color and the control point.



It's not the average, max, H, S, L, etc.



Anyone know?



Thanks,



--Rich Wagner

mask making