what I mean is, this sale is nearly as old as the last suggestion. The old suggestion had several sales to back up its value. If we're going to determine a common trading point around the time this sale happened, this sale would have been an outlier based on deviation from the mean.

This is a typical case where the arbitrary 3 month threshold would beat its purpose; in cases where values remain similar, I don't really mind I guess (though one could argue the significance of such a suggestion), but in this case, it means you're ending up pricing it based on an outlier sale because of the unfortunate timing.