"How exactly are the amounts entered on that form? It is very complicated and I am getting a formula that only taxes part of the 1/3 for 2021."

Having only 1/3 of the income reported for taxes in 2021 is exactly what you would expect, if the client took a distribution in 2020 and deferred the reporting for taxes over the allowable three years. Three years = 2020, 2021 and 2022.

How are you able to "get" something computing for 2021?

You are waiting for the Form 8915-F to be released by the IRS so that Intuit can update your ProSeries tax program. That is planned for March 17, according to Intuit.