The further out on the rail the slower it is to aim. The closer to the rear sight the faster it is to aim. Your index on the rifle will influence this a bit too. Think of it as a fine and coarse slider on a scale. Once out so far it begins to seem like you are wiggling the small weight around. This is an exaggeration of course but still the same principle. If you wanted to truly test it out I would start with some groups on paper with the sight at each end of the rail and see what the difference is. The ammo and mechanical accuracy of the gun will come into play too.