The Secretary shall provide for the issuance of wheat marketing certificates for each marketing year for which a wheat marketing allocation program is in effect for the purpose of enabling producers on any farm with respect to which certificates are issued to receive, in addition to the other proceeds from the sale of wheat, an amount equal to the value of such certificates. The wheat marketing certificates issued with respect to any farm for any marketing year shall be in the amount of the farm wheat marketing allocation for such year, but not to exceed (i) the actual acreage of wheat planted on the farm for harvest in the calendar year in which the marketing year begins multiplied by the normal yield of wheat for the farm, plus (ii) the amount of wheat stored under subsection (b) or to avoid or postpone a marketing quota penalty, which is released from storage during the marketing year on account of underplanting or underproduction, and if this limitation operates to reduce the amount of wheat marketing certificates which would otherwise be issued with respect to the farm, such reduction shall be made first from the amount of export certificates which would otherwise be issued. The Secretary shall provide for the sharing of wheat marketing certificates among producers on the farm on the basis of their respective shares in the wheat crop produced on the farm, or the proceeds therefrom; except that in any case in which the Secretary determines that such basis would not be fair and equitable, the Secretary shall provide for such sharing on such other basis as he may determine to be fair and equitable. The Secretary shall, in accordance with such regulation as he may prescribe, provide for the issuance of domestic marketing certificates for the portion of the wheat marketing allocation representing wheat used for food products for consumption in the United States. The Secretary shall also provide for the issuance of export marketing certificates to eligible producers at the end of the marketing year on a pro rata basis. For such purposes, the value per bushel of export marketing certificates shall be an average of the total net proceeds from the sale of export marketing certificates during the marketing year after deducting the total amount of wheat export subsidies paid to exporters. An acreage on the farm which the Secretary finds was not planted to wheat for harvest in 1965 because of drought, flood, or other natural disaster shall be deemed by the Secretary to be an actual acreage of wheat planted for harvest for purposes of this subsection, provided such acreage is not subsequently planted to any other price supported crop for 1965. An acreage on the farm not planted to wheat because of drought, flood, or other natural disaster shall be deemed to be an actual acreage of wheat planted for harvest for purposes of this subsection provided such acreage is not subsequently planted to any crop for which there are marketing quotas or voluntary adjustment programs in effect. Producers on any farm who have planted not less than 90 per centum of the acreage of wheat required to be planted in order to earn the full amount of marketing certificates for which the farm is eligible shall be deemed to have planted the entire acreage required to be planted for that purpose.