At the hearing, the commission may make such alterations in the assessments as in its opinion may be just or necessary to correct any error in the assessment list. The commission may increase or diminish any assessment as may be just and necessary to make the aggregate of all assessments equal to the total amount required to pay the entire cost of the work for which such assessments are made or the part of such cost to be paid by special assessment. No     assessment shall exceed the benefits as determined by the commission to the parcel of land assessed.