"Lot frontage" means that portion of a lot which abuts a public right-of-way or approved private right-of-way; provided, however, if certain portions of a lot abut two or more public rights-of-way or approved private rights-of-way, the frontage of the lot shall be the narrowest portion of the lot which abuts the rights-of-way.
(`78 Code, § 17.04.267.)