I'm going to restate this to see if I've got it.
If I've got more money coming out of the TIRA via RMDs than I want to spend, that money will have to be invested somewhere. There is no other option that's as attractive as a Roth IRA. But the money that came out due to the RMD isn't eligible for a Roth.
The key is to move it to the Roth before the RMD. I could wait and do that at age 69, but I might want to move so much in one year that I'd drive myself into a 28% bracket in that year. So it's better to do some math and see how soon I need to start to spread it out enough to avoid the 28% bracket.
But, that calc won't necessarily show that I need to be moving money at 60. Depending on my assets and other income, it might turn out that I can wait a number of years before I start.
well said; just remember, income and tax brackets/rates can change so do the math early and convert as early and as much as the math tell you to.