As used in this chapter: (1) "Benefit ratio" means: (a) for the period of January 1, 2011, through December 31, 2013, the number calculated by dividing the sum of all benefits charged to an employer during the forty calendar quarters immediately preceding the calculation date by the sum of the employer's taxable payroll for the same period. If fewer than forty but more than one calendar quarter of data are available, the data from those available calendar quarters shall be used in the calculation. The benefit ratio must be calculated annually using data for quarters filed through June thirtieth of the current year to the sixth decimal place; (b) from January 1, 2014, through tax year 2026, the number calculated by dividing the sum of all benefits charged to an employer during the twelve calendar quarters immediately preceding the calculation date by the sum of the employer's taxable payroll for the same period. If fewer than twelve but more than one calendar quarters of data are available, the data from those available calendar quarters shall be used in the calculation. The benefit ratio must be calculated annually using data for quarters filed through June thirtieth of the current year to the sixth decimal place; (c) For tax year 2027, the number calculated by dividing the sum of all benefits charged to an employer during the sixteen calendar quarters immediately preceding the calculation date by the sum of the employer's taxable payroll for the same period. If fewer than sixteen but more than one calendar quarters of data are available, then the data from those available calendar quarters shall be used in the calculation. The benefit ratio must be calculated annually using data for quarters filed through June thirtieth of the current year to the sixth decimal place; (d) Beginning in tax year 2028, the number calculated by dividing the sum of all benefits charged to an employer during the twenty calendar quarters immediately preceding the calculation date by the sum of the employer's taxable payroll for the same period. If fewer than twenty but more than one calendar quarters of data are available, then the data from those available calendar quarters shall be used in the calculation. The benefit ratio must be calculated annually using data for quarters filed through June thirtieth of the current year to the sixth decimal place; and (e) Notwithstanding the provisions contained in items (a) through (d), an employer who is in rate class one in tax year 2026 remains subject to the calculation contained in item (b) until the employer no longer qualifies for rate class one, at which time the employer shall be subject to the calculation in item (c) or (d), as appropriate. (2) "Department" means the Department of Employment and Workforce. (3) "Statewide average required rate" means the amount of income projected to be needed by the unemployment insurance trust fund for the upcoming calendar year divided by the estimated taxable wages over the same period rounded to the sixth decimal place. (4) "Statewide average interest surcharge" means the amount of income projected to be needed to pay interest on outstanding federal advances during the upcoming calendar year divided by the estimated taxable wages for the upcoming calendar year.