The guideline answers do not tax inflation. They also don’t say that the portfolio returns are non-taxable. I think that’s a contradiction.
Briefly, the Tracys need 4.5% after-tax income, inflation is 4%, taxes are 20% on withdrawal. I calculated the pretax nominal return as (4.5% + 4%) / (1 - 20%) but CFAI says it’s 4.5%/(1-20%) + 4% ( both in additive terms.)
I’ve been told that CFA Institute’s default position used to be that accounts were not taxable; I’ve been told that CFA Institute has changed their default position to accounts being taxable.
However, that’s moot here; make sure that you understand the vignette: Briscoe expects a tax rate of 20% to apply to the Tracys’ withdrawals from the investment account. You noted that yourself, above.
As I’ve said before, my experience with CFA Institute has been that they’re clear whether the account is taxable or not. Here, clearly, the account is not taxable.