At those income levels, the Child Tax Credit (CTC) may or may not be relevant to your marginal tax rate. That rate is what matters when deciding Roth conversion timing and amounts.
The CTC is, in general, a fixed amount so that wouldn't affect your tax rate. Depending on exact income, the split between the non-refundable and refundable portions of the CTC may come into play.
I'm still having trouble accepting (or understanding?) this broad rule that I've seen here and elsewhere on the forum that the marginal rate now vs. the marginal rate at time of conversion/retirement can always be used to determine Trad vs. Roth. At the very least it seems like the wording of the advice is either incomplete or not nuanced enough.
I've run about 60 potential scenarios through TaxSlayer (and ProConnect) to see what levels of Fed & state income tax are incurred when different amount of Roth Conversions are done ($20K, $30K, $60K, $80K) for different levels of earned income ranging from $10 up to $90K. I won't post the whole spreadsheet for now, because I think it will muddy the waters. But I'll post a couple of the scenarios below to help the question make sense.
OK, here are the constants for these scenarios:
Tax Year: 2020
Tax status: MFJ
Children under 16: 3
Net Sch E income: $600
Ord dividends: $2700
Qual dividends: $1700
Taxable interest: $300 (Total of $5300 investment income, which does not disqualify the earned income credit)
Earned income: $10
Note: I believe my state allows the first $30K in retirement income (including conversions) for each person to be done tax free.
Scenario 1:
Roth Conversion of $60K
Fed Marginal Rate: 12%
State Marginal Rate: 0%
AGI: 63610
Taxable Income: 38810
Child Tax Credit: 4060
Earned income credit: 0
Add'l Child Tax Credit: 1
Total Fed Tax: $-1 ($1 refund)
Total State Tax: $0
Fed tax attributable to Roth Conversion: $10 (.02%) (attributable is higher than actual, because the EIC was lost)
State tax attributable to Roth Conversion: $0 (0%
Total tax attributable to Roth Conversion: $10 (.02%)
Total tax saved by Conversion: $10,190 on $60K = 16.98% (assuming original 17% marginal when deferred)
Scenario 2:
Roth Conversion of $80K
Fed Marginal Rate: 12%
State Marginal Rate: 5%
AGI: 83610
Taxable Income: 58810
Child Tax Credit: 6000
Earned income credit: 0
Add'l Child Tax Credit: 0
Total Fed Tax: $460
Total State Tax: $839
Fed tax attributable to Roth Conversion: $471 (.59%) (attributable is higher than actual, because the EIC was lost)
State tax attributable to Roth Conversion: $839 (1.05%)
Total tax attributable to Roth Conversion: $1310 (1.64%)
Total tax saved by Conversion: $12,290 on $80K = 15.36% (assuming original 17% marginal when deferred)
So for the person considering this scenario: (That's me, with 3 kids and the opportunity to have only $10 earned income and do a Roth Conversion in the range of 60-80K)
These things are known:
At the time that we deferred this income into our Traditional IRA/401(k) our Fed marginal rate was 12% and State was around 5-6%
According to both Taxslayer and ProConnect:
Scenario 1: Roth conversion $60K
Fed Marginal rate: 12%
State Marginal rate: 0%
Scenario 2: Roth conversion $80K
Fed Marginal rate: 12%
State Marginal rate: 5%
Extra note: the difference in total tax on Scenario 2 vs Scenario 1 = $1300 (of an extra $20K converted to Roth) = 6.5%
Of that 6.5%, 4.2% is state which is expected because of the first $60K not taxed, and the remaining 2.3% is Federal.
So the additional $20K in conversions only incurred 2.3% in federal tax, despite the marginal rate being listed
as 12% for scenario 1 and scenario 2.
I think most people heeding the advice about the "marginal tax rate being what matters" would assume that if scenario 1 ($60K conversion) was known to already have a 12% marginal rate, then any additional conversions(or any taxable income) being considered would be expected to incur a 12% (federal) tax. Obviously, in these 2 scenarios, staying closer to $60K in conversions will be the most "tax efficient" with almost all of it incurring no state or federal tax. And I'm sure it's possible to determine the max amount that can be converted so that all $6K of the child tax credit can be realized. For a person with enough savings and time, it probably makes sense to stay more efficient.
At this point, I'm not necessarily looking for advice on whether to plan for Roth Conversion in general. The spreadsheet of scenarios I've built shows me how much I can "save" by doing a certain Roth Conversion amount at a certain earned income level, using the constants above. As others have pointed out, the earned income credit is a factor in some of these scenarios and it's most beneficial with 3 kids... which means it's harder to justify Roth Conversions when it's in play because conversions of any significant size make it unavailable. Anyway, what I'm looking for is some clarification or update to the general advice about marginal rates being "what matters" in these situations. Is there a general rule that will always be true? Or will it always be the case that it's always necessary to run the numbers so see? I only know my situation, which involves the child tax credit x 3. Maybe there are other credits that have this same effect on the general rule??