When you see a paint sample strip that has 4-6 colors on it that seem to be gradations of "shade," is there actually any rule of thumb for the percentage difference between the "shades?" I'm getting a lot of customers lately who pick a primary color for a room and then ask me to paint an "accent" wall "one shade darker." My paint store guys kinda shrug their shoulders and tell me they can do like 25% or 50% of any color on a strip....but the result doesn't usually end up being close to the next color up on the sample strip. I'm thinking that "shade" is subjective and the customer must do final approval of what I might suggest to be "one shade darker." Do any of you use the 25% darker or lighter rule?