Thank you, that's very helpful!
Just to make sure I'm parsing everything correctly, you're saying that if my liability on non-self-employment tax plus capital gains are reduced by credits enough that the the excess withholding is enough to cover the Self-employment tax amount, I'm okay.
That is, in very simplified terms, no additional withholding or estimated payments are necessary if:
[withholding] - [W-2 income tax] - [capital gains tax] + [nonrefundable credits] > [self-employment tax due]
