After hearing all objections and evidence, the board of supervisors shall:
(a) Add to or deduct from the valuation assessed to any parcel of land a percent thereof sufficient to raise it or reduce it to its full cash value.
(b) Fix the full cash value of any land that has not been assessed.
(c) In the case of lands assessed pursuant to Section 36577, change the assessed value of any parcel so as to reflect the proportional benefit bestowed upon the parcel by the operations of the district.