Texas Government Code - GOV'T § 803.302. Computation of Certain Benefits

(a) If payable to or on behalf of a person who has used combined service credit to qualify for benefits from at least one retirement system to which this chapter applies, each of the following types of benefits must be computed as provided by Subsection (b):

(b) The amount of a benefit payable under Subsection (a) by a retirement system to which this chapter applies is a percentage, but not more than 100 percent, of the benefit that would be or would have been payable if the person retired or had retired on the basis of only the service that is credited in that system. The percentage applied is equal to the amount of service credit in that system, divided by the amount of service credit that would be or would have been required for the benefit if the person retired or had retired on the basis of only the service that is credited in that system.