How soiled is too soiled for hatching?

How do you decide if your egg is too dirty too hatch? I have one egg that had some manure on it and tried removing as much as possible with fine grit sand paper but you can still see a slight stain to the shell, but not even half of the the shell had it on it. Whats your opinion on what is considered too soiled of an egg. Any pictures you may have to help clearly show would be great. Thanks in advance