Roth Conversions have become very popular and I think they deserve to be treated as "first class citizens" in the data entry modeling.
Other threads in this forum are ended with the suggestion to model it with 3 line items in the advanced inputs.
- - Tax Deferred
- + Tax Free
- + Other Expense (for taxes)
Also, noted are the problems with this.
- The tax rate that should be used
- running out of funds in Tax Deferred - causing the + Tax Free to overestimate the amount in savings.

I think another "Cashflow Type" of "Roth Conversion" would help a lot. It would use at least the start year, end year, and annual amount. Tax would be computed like any other expense, the tax rate would be like any other income, and the Roth Conversion would not happen if the Tax Deferred ran out of funds before the end year arrived.
I have had to spend considerable amounts of time trying to

guess what these numbers should be to avoid the issues and get a reasonable level of conversions over a multi-year period. Especially when I change the retirement year, or change the amount of 401k contributions, this becomes very difficult to keep up to date.
This may not be a perfect, or even good, solution

, better minds can chime in!