an extension payment will not affect TurboTax's calculation of underpayment of estimated tax penalty.
this occurs when you owe more than $1000 and your withholding and timely estimated tax payments (an extension payment is not an estimated tax payment) don't equal or exceed a) 90% of your current year tax or b) 100% of last year's tax (110% if your prior year adjusted gross income is $150K or more).
the penalty is calculated using the lower of a or b.
