Keep in mind that the auto appraisal should be used as a suggestion first, then weighed as advice. As it is auto it is based on calculations, which as in this case, can be, at times, flawed.
As far as a formula goes, you can try setting a custom one, depending on what specs you value the highest, but the default will suffice. Especially true if you are working the drop lists, as most the great domains are being 'tasted' by the super squatters (registrars) for traffic anyways.