/ Income Taxes: Turbo Tax - H&R Block - File Your Own? #11
I've used a spreadsheet from the site below for a good number of years and it's been accurate.
Federal Income Tax Spreadsheet Form 14 (Excel Spreadsheet) Income Tax Calculator
Once you get the hang of what numbers go where, it pretty much fills the rest out for you. Libre Office works with it if you don't have a newer version of Excel.
This is impressive.