Penalty calculation on falling short of 90% of tax liability.

Suppose my total federal tax for the year is $100,000. To avoid the underpayment penalty under the 90% rule, I need to make timely estimated payments of at least $90,000.

If I make four timely equal payments totaling $90,000 or more, there is no penalty.

But if I make four timely equal payments totaling only $89,900, I have missed the 90% safe harbor by $100.

Is the underpayment penalty calculated on:

1. the full $100,000 tax liability,
2. the unpaid balance due at filing, which would be $10,100, or
3. only the $100 shortfall between what I timely paid, $89,900, and the 90% required amount, $90,000?

I am pretty sure the answer is #3, but I am hoping for confirmation from people who 110% confident, whereas I am only 89.9% confident. (haha)