Do I owe an underpayment penalty for 2019 if I owe $7800, but my withholding for the year was only $6500 - so below 90% of what I owe. Turbotax says no penalty.

According to the IRS, you can avoid the underpayment penalty by comparing the tax owed on your 2018 return. If the withholding and estimated tax paid during 2019 is 100% of the tax shown on your 2018 return, then you do not owe an underpayment penalty,