You don't need to calculate the earnings on an excess contribution to your HSA; let the HSA custodian do it.
When you call the HSA custodian (or do the form online) to report the request to withdraw the excess contribution, this will start a process at the HSA custodian. They will calculate the amount of earnings, and then send you the excess plus the earnings out of the HSA. This will be reported on a form 1099-SA.
Note that for 2020, the excess contributions will be added to line 8 of Schedule 1 (1040), if the excess was due to the amount with code W in box 12 on your W-2. So the excess is taxed in 2020.
However, you will not get the 1099-SA with the earnings until next year, to add it to your 2021 tax return.
So, ask the HSA custodian to withdraw excess (specifying the excess amount only); they will take care of the earnings - since some of the earnings belong to 2021, the IRS lets you defer taxes on all the earnings until 2021.
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