Underpayment Penalties

Have you read through the instructions to the form?

2019 Instructions for Form 2210 (irs.gov)

Or are you asking about the late payment penalty that starts with the return due date when an extension is filed but without enough tax paid with it?

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@johnicahorton wrote:

What can anyone tell me about how the penalty for not having paid in 90% of tax works (as distinct from penalty for not paying quarterly estimated tax payments)? 

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It is not distinct from the estimated tax penalty.  They are the same thing.

If a taxpayer did not pay at least 90% of what they owe (see note below), they SHOULD have made Estimated Tax Payments (or had higher withholding).  So that is the Underpayment of Estimated Tax Penalty.

As you read the instructions, it is more complicated than that.  Factors include if you owe more than $1000, what last year's tax was, and higher income taxpayers may need to pay more than 90%.  But the bottom line is that you are referring to ONE penalty, not two different ones.

Pull Form 2210, and walk (very carefully...) thru page 3 & 4.  It should clarify things much better than words can.  It is actually very logical.

Another approach is to use a dummy client, make it have a liability, then play with the numbers coming from withholding and from estimates.  Change the dates on the estimates (make some late, or even zero) and review the F 2210 page 3 & 4 and worksheets until you understand the impact.