That would work approximately except I don't think it will be exact. Your will boil off a larger % of the wort I would assume and your beer will Carmeliize more giving you a diff color. Hop utilization will also differ. You might boil off only 1 gallon in a five gallon boil but still boild off nearly as much in a one gallon batch. The boil off rate is closely related to kettle diameter and boil vigor. Just my two cents.