Photoshop :: Single Number For Estimate Of Grayness

I'm playing around with images of projector screen fabrics and want a single number that indicates how close to being neutral the image is. What I do is select a small portion of the image, average the selection, and use Photoshop to tell me the color numbers. Let's just say the Lab numbers (L, a, b) for three fabrics are:
 
A: 80, -2, -6
B: 85, 7, -1
C: 83, -4, -4
 
The L gives me the luminance (how bright the screen is), and I think the "a" and "b" should be zero for a true neutral. But if they are not zero, what is the best way to indicate how far the color is from gray?
 
Use an absolute average? In which case the single number, call it "grayness" (with 0 being true gray), would be 4 for all samples, indicating that the fabrics are equally gray.Use Pythagoras to calculate the distance from the a,b origin (assuming "a" and "b" can be treated as perpendicular axces)? In which case the "grayness" value would be: A = 6.3, B = 7.1, C = 5.7, indicating that C was the closest to gray.Other method.