The Secretary shall proclaim a national acreage allotment for each crop of wheat. The amount of the national acreage allotment for any crop of wheat shall be the number of acres which the Secretary determines on the basis of the projected national yield and expected underplantings (acreage other than that not harvested because of program incentives) of farm acreage allotments will produce an amount of wheat equal to the national marketing quota for wheat for the marketing year for such crop, or if a national marketing quota was not proclaimed, the quota which would have been determined if one had been proclaimed.