Thanks @SecondCor521, you've echoed my thoughts. And TurboTax, which says: Follow these steps to calculate the percentage of your distribution that qualifies:
1. Add the amount of your 2022 distributions to the total account balance as of December 31, 2022.
2. Figure the total of all U.S. government interest received by your IRA or retirement account from the beginning through December 31, 2022.
3. Subtract the total exempt part of your distributions from all prior years through 2021. If the result is negative, use zero for this step.
4. Divide the result from Step 3 by the result from Step 1. This is your Oregon exempt ratio.
5. Multiply the result from Step 4 (Oregon exempt ratio) by the amount of your 2022 distributions.
When I contacted Vanguard it was to ask them if they could provide this history, as they've held my IRA for 20 years. Given how sophisticated technology has become in so many ways (AI chatbots writing term papers!), I would think a financial institution like Vanguard could do the math for me. Then again, I suppose there' nothing in it for them.
This is the first year I've taken a distribution (an IRA rollover), thus the first time this has come up. The distribution was large enough that I figure the state-level subtraction could save me a couple hundred bucks.
As best I can tell, it'd be impossible to calculate exactly since the full data just aren't available. Guess I'll swag it, and assume that my AA has been pretty consistent over the years (35% fixed income funds) as have the % of income in those funds from government obligations (using Vanguard's 2022 data as a proxy).
Going forward, I may start keeping track of this each year. In hindsight, that would really be the only way one could have the data, if you'd kept track each year since the beginning. I do have almost 30 years of Quicken data for my checking accounts, but sometimes didn't keep up with portfolio tracking due to technical challenges with importing the data, etc.