One of the steps for our regular test for our GraCol7 is to measure the 50 percent black with a densitometer. We are suppose to read a density of 0.50 plus or minus 0.1 I believe is the spec. Prepress says we are suppose to measure the paper density and add that to the density reading of the 50 percent patch. That does not make sense to me. I would think you would subtract the paper density from the reading. So which is correct say the patch reads .51 and the paper reads .05 is the correct reading .56 or .46?