The Potential Revenue Index (PRI) emphasizes the per cent of current school revenues that are derived from unequalized local taxation. Its purpose is to measure the extent to which potential school per pupil revenues are equalized by various State school support programs. One can also ask how much additional tax revenue people in low wealth school districts would need to pay to raise their school's per pupil revenues to the State average. The proposed PRI formula for measuring financial equalization uses variables based on the per cent of the State per pupil average expenditure rate attainable in the "key" low wealth school district if it levies the State average supplemental school tax rate. The formula calculations are explained and applied. (Author/DW)