I wouldn't worry about a possible 3mm difference given the distance between the cog and the crank. The offset angle is going to be very small. Very small. Today's chains won't care a bit.

And who wants to carry a ruler with them so every time they change a tire or flip the wheel they can be absolutely sure they get the chainline in a perfect 42mm?


Way Off topic: saw this kid walking to school yesterday. Freezing cold but since he's in high school he can't be bothered to wear a coat. Had the long hair in the eyes so he can't see where he's going. I had to stop for traffic so I roll down the window and ask him: "being emo sometimes sucks, doesn't it?" He just looked at me with that 'I just swallowed some of my mom's valiums because life is too hard' expression and shuffled on down the road.