The Safe Harbor not only has the 90% rule, the 100% rule, the 110% rule. It also has a timeliness requirement. The safe harbor needs to have been paid by any combination of withholding and estimates. Estimates need to be timely. Withholding is considered to be "for the year" and not by quarterly. This is one reason I put seniors with investment income on Social Security withholding, so they can meet safe harbor and not worry about missing estimate due dates.