Tex. Tax Code § 151.416
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:
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.