Gum Calibration 2 (How to read color samples?)

[Loris Medici <mail@loris.medici.name>]

This goes to those who calibrate with PDN / ChartThrob and use a scanner
for making readings from the printed step tablet:

What adjustment(s) you do, to the scan of the step tablet? Do you
desaturate it only?

I will re-calibrate from scratch (and separately for each color), using
stock paint:gum solutions, that's why I'm asking...

I was thinking to change the image mode to Lab, split channels, and then
use the Luminosity image (mode changed to RGB) for taking the readings.
That way, the basis of the calibration would be Luminosity. That sounds
right to me, since we're trying to match both chroma and luminosity for a
good color balance, isn't it?

I just don't like the idea of desaturating, since it's not obvious how
Photoshop does it, and feel like it can give quite different results
depending on color... (I'm not sure saying that -> just a feeling ->
doesn't seem right.)

Any thoughts?

Thanks in advance,
Loris.