How do you get gray scale measurements in black and white percentages in Photoshop?

I'm trying to count the intensity and distribution of dark spots on a surface using Photoshop Cs2 and I need to be able to compare between the different surface "darknesses". I'm doing this by blurring the whole area to get the % darkness but I don't know how to go from CMYK/RGB to a one value number which is easier for compariosn. Please help me asap!!!