I know there are some who are in this situation. Assume you can't use previous yr safe harbor because last yr was a high income yr. If you were trying to meet 90% of this yr taxes as safe harbor and then Q4 was higher than you expected............is there a simple rule of thumb to figure out what to pay in Q4. First 3 quarters payments were equal and likely ? ahead for that period because income is normally higher in Q4. I am wondering if shooting for 90% of annual taxes total est. taxes and subtracting out Q1-Q3 payments would get me close since I'm probably ahead for the first 3 quarters.