You've finished brewing and your wort is in the fermenter. What's the most your measured OG can deviate from the estimated (according to the recipe, Beer Smith, etc) and you still feel like you've done a good job?
It really depends on the beer. I'd be way less concerned with a 1.105 on a planned 1.110 barleywine than a 1.030 on a planned 1.035 mild.