Pick a year and a limit. Each input is circled on the county's certified report, labeled A1, A2 ... for Travis CAD and B1, B2 ... for Williamson CAD. Then comes the simple math, then each line of the Comptroller's worksheet. At the end you can download the official form filled in with these numbers. Williamson-Travis Counties MUD No. 1 is the example; choose "Your district" to start blank.
Four numbers per county per year: the homestead count, the average market value, the total homestead cap (the 10% limit on a homestead's appraised value, Tax Code 23.23), and the average taxable value. "Prior year" figures come from the current year's certified report, where the county restates last year. Inputs the chosen method does not use are dimmed. You can type over any value; changed cells turn amber.
The statute asks for the "average appraised value of a residence homestead in the district", disregarding the exemptions for people 65 or older and people with disabilities (Water Code 49.23602(a)(4)(C), 49.23603(a)(3)). Appraised value is the value after the homestead cap (Tax Code 1.04(8), 23.23). Neither county prints that average, so it is built here. The statute does not say how to combine two counties.
For each of the three prior years: that year's voter-approval rate (A), its unused increment rate (B), and the rate actually adopted (D), times that year's total taxable value (F). A year adds revenue only when A minus B is more than D. Leave zeros if you do not have the figures; the form's "current total value" (Tax Code 26.012(6)) divides the sum.