for them since their 2019 adjusted gross income was over $150,000 as one of the options they would need to pay in 110% of their 2019 taxes not 100%
it is also entirely possible that with $300,000 in short-term capital gains for 2020 they could end up owing more than if they had $400,000 in long-term capital gains for the same year.
within the TT program for 2019, there should be a section where they can enter their estimated 2020 amounts to get a better idea of where they stand. some taxpayers like to owe as much as possible without penalties, others prefer to break even and there are those who want to get a huge refund - that's how they save for a vacation or large purchase - let the IRS hold their money interest free.
