The statewide average expense per pupil, for purposes of subparagraph 1 of paragraph a of subdivision 5 of section 3602 of the Education Law, shall be determined using year prior to the base-year-approved operating expense and pupil counts. The commissioner will prepare and estimate the data for each district which has not submitted such data by such date. The statewide estimated approved operating expense for the base year will then be divided by the statewide estimated pupil counts for the base year, and the result rounded to the nearest $50, to arrive at the statewide average approved operating expense per total aidable pupil unit.