you should basically aportion all the income items which result in your AGI so anything on your 1040 in Lines 1-10 which includes the HSA adjustment on line 10. Check also the instructions for Form 2210: "If you use the cash method of accounting (used by most people), include all income actually or constructively received during the period and all deductions actually paid during the period."
I went thru this for same situation, I went thru the interest, dividends, cap gains etc line by line to build that view in Excel. You may find for some income that the result is not materially different from just doing a pro-rata if that income flow was steady thru the year. Maybe your net capital loss -3k was actually made up of some gains and losses in different quarters which may create some uneven AGI, possibly causing other penalty situation if say the gains occurred earlier in the year. Depends on your situation and how material it all is vs. the Roth and Q4 estimated tax I assume you are trying to eliminate the penalty on.
Also check your state requirements for similar process before you go thru this - after doing this for Fed I found out VA asks for AGI thru 4/30, 5/31, 8/31 instead of 3/31, 5/31, 8/31, so I had go thru it all again to split out April and May (which makes no sense, I don't know why can't align with Fed).
It's a bunch of work but I was able to eliminate the penalty for 2024. For 2025 I plan to pay quarterly estimated tax based on 100% of 2024 and not have to deal with it again regardless of Roth conversions.
Not a CPA/Expert - just my 2 cents - hope this helps.
