I like Tom's answer and was about to make the same point: $420,000 still could be too much. Get the comps and see what its real value is. We know it's not $460,000, or else it would have sold. You may be assuming $440,000, since the American notion of "fair play" seems to be "split the difference." No knock on you--and for years I'd always gone with "split the difference" too. But that's not always the best approach.
Anyhow, determine the real value. Even if the comps suggest it's above $420,000, make your offer. And if it's close or below, lower your offer accordingly.