@user17609754020
The safe harbor means that, you must pay into the system in 2025 (by withholding or payments) 100% of the tax you owed in 2024.
For example, if in 2024, you paid $12,000 in withholding and got a $2000 refund, the tax you owed to the IRS was $10,000. You must pay at least $10,000 to the IRS in 2025 to qualify for the safe harbor. Your actual tax for 2024 is shown on form 1040 line 22 and does not include self-employment tax or penalties.
However, I am not convinced this will protect you. It is a safe harbor for the general underpayment penalty that is assessed if you are short when you file your return, but I don't think it protects you from the under-payment penalty that you will owe back-dated to April 15, June 15 and Sept 15 for the estimated payments due from the lump sum of income. I still recommend you make an estimated payment by January 15 and include form 2210 and Schedule AI with your return.
