I photograph fine art for giclee reproduction using different greycards for adjusting whitebalance and brightnes. White balance is never a problem, but brightness is. First of all, the color values for the white/grey/black patches on the greycard is not compatibloe With the way LR calculates the rgb% values. I have read that the percentage values in LR are based on ProPhoto RGB chromaticity values but with an sRGB TRC gamma curve (2.2). The color values I have found for X-rite ColorChecker, Datacolor SpyderCheckr and QP Card is L-a-b, Adobe RGB and sRGB, but never ProPhoto RGB. I have asked for supliers support with no luck so far. Is there any converter or formel available for converting to and from LR way of calculating rgb% values?
I usually start with adjusting exposure to meet the values for the whitest patch, then adjusting blacks (or shadows) to meet the values for the black patch. The result seem ok for the whites and blacks, but the medium shades seldom fit with the values for the grey patches. Is there there a better workflow for doing this? Should I start adjusting exposure to meet the values of the grey medium grey patches?
Set Lightroom to show Lab values (right-click the histogram). Then see what the corresponding % is.
My rule of thumb is that the third darkest patch in the ColorChecker should be 48%, the lightest 93%, and the darkest somewhere around 10%. That's as firm as I want to be, because some final tweaking is always necessary to produce a credible match. But those figures get me in the right neighborhood quickly.
Make sure you have your display calibrated to white and black points that actually match your output. This is just as critical for a good result.