Thank you for the reply. Just to clarify, I run a business (an LLC) and am trying to decide whether to add to my HSA by writing a personal check (not a business check) before April 17th. It looks like I would gain nothing from a tax standpoint by doing so.
If your income tax is already zero, making an HSA contribution will not further reduce your income tax below zero. An HSA contribution will not reduce other types of taxes such as your self-employment tax.
The total Tax Due is about $13,000, but since I run my own business, they are SE Taxes. Guess I didn't realize one could not offset those taxes. I noticed a similar result (no change in taxes) when I tried to "model" making 401K contributions. Presumably they too, cannot offset SE taxes.