Tex. Tax Code § 151.416: COMMINGLED RECEIPTS AND TAX.

A seller who has an accounting system under which the taxes collected under this chapter are commingled with the receipts from the sales of taxable items may compute his taxable receipts by:

(1) subtracting from the total receipts of the seller the receipts from the sales of items that are exempted or are specifically excluded from the taxes imposed by this chapter to obtain a remainder consisting of the commingled receipts from taxable sales and the taxes collected; and

(2) dividing this remainder by one plus the sales tax rate expressed as a decimal fraction to obtain a quotient that is the taxable receipts that may be reported under Section 151.410 of this code.

Acts 1981, 67th Leg., p. 1573, ch. 389, Sec. 1, eff. Jan. 1, 1982. Amended by Acts 1984, 68th Leg., 2nd C.S., ch. 31, art. 13, Sec. 4, eff. Oct. 2, 1984; Acts 1986, 69th Leg., 3rd C.S., ch. 10, art. 1, Sec. 3, eff. Jan. 1, 1987; Acts 1987, 70th Leg., 2nd C.S., ch. 5, art. 1, pt. 3, Sec. 3.