The Administrator will allocate to an opt-in unit an amount of allowances equal to fifty percent  the lesser of the unit's baseline heat input or the unit's heat input for the year before the year for which the Administrator is determining the allocation; multiplied by the lesser of the unit's baseline emission rate, the unit's 2002 emissions rate, or the unit's most stringent State or federal emission limitation applicable to the year on which the unit's baseline heat input is based.