How should we calibrate against SOI targets post-TCJA?

[MaxGhenis]

Starting in 2026, more tax units will have to file due to TCJA's expiration. CBO's projections account for this, but lack the distributional information of SOI reports. However, the change in filing requirements means we should not merely index the SOI numbers by the population or income, especially targets for taxable returns.

Relatedly, we may not want to assume the same distribution of returns as today, given income growth should push more into higher bins. This assumption will distort increasingly as we project into future years.

Overall, we may want to consider structural approaches to indexing SOI targets. @nikhilwoodruff @donboyd5 @eqberkovich