rule for when going from joint to single
first, compute the tax that both parties would have paid in 2019, based on their 2020 filing status. then multiply the tax shown on the 2019 joint return (110% if joint AGI for 2019 is over $150,000) by the following fraction
the tax you would have paid in 2019 based on your 2020 filing status dividend by the sum of the taxes that both parties would have paid in 2019 based on their 2020 filing status
example spouse A spouse B
2019 taxable income $20 $10
2019 tax using 2020 filing status - single $6 $4
2019 joint return tax $5 (AGI under $150,000)
Spouse A share $6/$10 * $5 = $3
Spouse B share $4/$10 * $5 = $2
what isn't clear is what to do if one taxpayer has self-employment tax and the other doesn't. as a guess additional taxes and tax credits should be allocated to the specific taxpayer that created or caused them.
certain credits and additional taxes may need to be split.
the other exception is for you to have timely paid 90% of your 2020 tax
you can use the annualized income schedule on form 2210 to see if that lowers or eliminate the penalty
https://www.irs.gov/pub/irs-prior/f2210--2020.pdf
see page 4
