(a) Except as provided in Section 58.512(b)(4) of this code, before the commission of appraisement is appointed and the construction bonds are sold, the board shall adopt a proposed plan of taxation as provided in Sections 58.502-58.505 of this code.
(b) If the tax plan is not based wholly on the ad valorem basis or on the benefit basis, the order adopting the proposed plan shall specify the portion of the tax to be based on the ad valorem basis and the portion to be based on the benefit basis. The board also shall state the physical and economic reasons, the peculiar diverse local needs, or the comparative potential benefits of different areas of designated property in the district which make it necessary or equitable to levy all or part of the tax on a defined part of the district on the ad valorem or benefit basis.