@ghoofie - I thought the same thing when I saw the $300 on Line 12b on my 2020 Federal Tax Return. But then I checked on the IRS site and learned that the Charitable Donation addition on Line 12B was $300 for ALL filing statuses in Tax Year 2020. The change to the $600 Charitable Donation addition for Married Filing Jointly occurred for Tax Year 2021.
