Tex. Educ. Code § 49.157

The total amount required under Section 49.153 for a district to purchase attendance credit under this subchapter for any school year is reduced by an amount equal to the product of the district's total costs under Section 6.06, Tax Code, for the appraisal district or districts in which it participates multiplied by a percentage that is computed by dividing the total amount required under Section 49.153 by the total amount of taxes imposed in the district for that year less any amounts paid into a tax increment fund under Chapter 311, Tax Code.