"Rate of insured unemployment" for purposes of section 23-630 means the percentage derived by dividing:

1. 1. The average weekly number of individuals filing claims for regular benefits in this state for weeks of unemployment with respect to the most recent thirteen-consecutive-week period determined on the basis of reports submitted by the department to the United States secretary of labor, by
2. 2. The average monthly employment covered under this chapter for the first four of the most recent six completed calendar quarters ending before the end of such thirteen-week period.