married couple, both 66, sitting on $1.5 million in a traditional 401(k), Social Security deferred, and roughly $10,800 of other taxable income from a part-time consulting gig and a slice of interest.

Converting exactly $90,000 fills the 12% federal bracket to its $100,800 ceiling, avoiding an 83% marginal rate jump on every dollar above it. Paying 12% now on $360,000 converted over four years ...