74On or before the first day of April in each year, or so soon thereafter as the Commission may deem expedient, the amount of the assessment for the previous calendar year shall be adjusted upon the actual requirements of the class and upon the correctly ascertained payroll of each industry, or otherwise as the case may require, and the employer shall forthwith make up and pay to the Commission any deficiency, or the Commission shall refund to the employer any surplus, or credit the same upon the succeeding assessment.