It looks like the rules for estimated tax and safe harbor are different.
My income last year was less than 150k.
If I pay 110% of the tax that I owed last year, would that mean I would be in Safe Harbor?
Estimated tax safe harbor for higher income taxpayers. If your 2017 adjusted gross income was more than $150,000 ($75,000 if you are married filing a separate return), you must pay the smaller of 90% of your expected tax for 2018 or 110% of the tax shown on your 2017 return to avoid an estimated tax penalty.
Who Must Pay Estimated Tax
If you owe additional tax for 2017, you may have to pay estimated tax for 2018.
You can use the following general rule as a guide during the year to see if you will have enough withholding, or if you should increase your withholding or make estimated tax payments.
General rule. In most cases, you must pay estimated tax for 2018 if both of the following apply.
You expect to owe at least $1,000 in tax for 2018, after subtracting your withholding and refundable credits.
You expect your withholding plus your refundable credits to be less than the smaller of:
90% of the tax to be shown on your 2018 tax return, or
100% of the tax shown on your 2017 tax return (but see Special rules for farmers, fishermen, and higher income taxpayers , later). Your 2017 tax return must cover all 12 months.