You are right. I failed to mention that we are a family of 4, about to become a family of 5. For us the 400% FPL is a little over $110k, so I was honing in on the top of the 0% tax bracket as it is lower. Thanks for pointing out the standard deduction. It is amazing how comfortable I feel with investment stuff and how utterly clueless I am on the tax side. Time to fix that.
As for balancing Roth conversions vs ACA subsidies I guess we should just run some scenarios and see what feels better?
Ah, yes, having a larger tax family size, as the ACA and IRS so poetically put it, changes the relative AGI levels. As well as where you live. The link I provided for FPL in my previous post is one of the clearer explanations of that part of it that I have found; I would refer you there.
Balancing Roth conversions and ACA subsidies, there are a number of things to consider, depending on how complicated you want to get:
1. All other things being equal, Roth conversions reduce ACA subsidies between ~138% and 400% of FPL.
2. There are actually three (I think) different graduated rates in the above range. So Roth conversions between, say 138% and 150% reduce your ACA subsidies at a more rapid rate than between 300% and 400% of FPL. You can see this by playing around with Form 8962 or doing careful math with the "Applicable Figure" in Table 2 of the Form 8962 instructions.
3. Roth conversions increase AGI and taxable income, so there are other effects of Roth conversions beyond ACA subsidies. Using my particular case as an example, the increase my state income taxes, affect my FAFSA results two years from now, and affect AGI eligibility for other tax benefits (like the retirement savings tax credit).
4. On a very long term view, Roth conversions help avoid what is called the tax torpedo. Basically, if you're FIRE, then when you hit 70 and are collecting Social Security and taking RMDs from your traditional IRA, it can push you into a high tax bracket. So what I do is figure out what bracket I'll be in when I am 70, and then try to do my Roth conversions now so that I shift forward some of that higher-when-I'm-70-taxed-money to now. This is a hard to impossible thing to get exactly right, but it's probably worth thinking about if you think you'll live past 70 and if you think the tax rules will be approximately the same as they are now.
5. In a shorter term view, you want to make sure you Roth convert enough to live on five years later (to the extent that you're living off your Roth conversion ladder).
6. With dependents, the child tax credits and other dependent tax credits are non-refundable. This means that you have to create a high enough income to create a high enough tax bill to use up all of these non-refundable credits; otherwise they are "wasted". Personally I always Roth convert enough to make sure I don't leave any of those credits on the table.