both Alabama and IRS rules are the some for avoiding underpayment penalties
no penalty if on a timely basis, your withholding and estimated tax payments are the smaller of the following two amounts
1) 90% of 2020 tax
2) 100% of 2019 tax (110% for IRS if 2019 adjusted gross income is more than $150,000
also
3a) for the IRS if 2020 tax less withholding is less than $1000 no penalty
3b) for Alabama if 2020 tax less withholding and estimated tax payments is less than $500 no penalty.
it is likely exception 2 above would be the lowest for the amount of taxes that have to be paid in through withholding and estimated tax payments. meeting this exception means you could owe $1,000,000 come tax time and still not get hit with a penalty. it's up to you as to whether to increase estimated tax payments to cover the STCG if you will be safe from penalty under 2) above. some people just do not like to owe come tax time.
