Digital BW, The Print

I have been working to create ICCs for several combinations of paper and ink that I am using. I have a question regarding the correct way to read the values off of a printed test step scale. 

My methodology has been to print a 21-step scale with the candidate ICC and then scan it using an Epson flat bed scanner. I then open the scan up in Photoshop and read the scales using the information panel. 

Recently, I began including a Kodak gray scale card along with my printed step scale in the scanner. When I read the values from the Kodak card in Photoshop they do not equal what they should be. 

My question is: Should I linearize the image using levels or curves so that the Kodak scale reads correctly before I begin reading my printed step scale? Would this correctly adjust for any off-set I may have in the scanning process?