Paul W, I saw that you posted an updated PDF about your spreadsheet method on the QTR group and was going to respond there too. Are you aware that there is a Photoshop script that can be downloaded from the QuadToneRIP website that will do automatically do the same thing as your spreadsheet method? The QTR-Stepwedge tool uses the Q-13 target as a reference and adjusts the measurements of the scan based on the known measurements of the Q-13 target. It then goes through each patch on the printed 21 step target, averages a selection of each patch to blur the paper surface and any irregularities, gets the LAB values for each patch, and then exports a text file with the measurements. I tested it a long while ago and it does a pretty good job, and it is much easier than measuring and copying each step by hand.
If you are set on measuring by hand with Photoshop, another thing that you can do rather than messing with the RGB values is just change one of the alternative measurement displays in the info pallet to show the LAB data. I set the readout to display 32-bit values, and they are usually pretty close to what I have measured with an i1 Pro. From there you can type them into my correction curve tool or the linearize= line directly or you can can just use put them into a text document and drop it onto the QTR-Linearize-Data app, which will spit out the density conversion and give you the string to pop in the ink descriptor file.
Richard Boutwell
http://www.richardboutwell.com/