HELP: I can't get my spreadsheet calculated taxes to match TurboTax

I have a spreadsheet that I use to calculate my taxes using the Married Filing Jointly brackets.  I have verified in the past that my math is correct (I'm pretty good with spreadsheets).

 

However, I'm ready to file my taxes and ran a cross check only to find my spreadsheet numbers don't match TurboTax 1040 Line 16 ("Tax").  Of course I assume TurboTax is correct, but I want to predict next years taxes so I need to get my spreadsheet is working correctly.  First, I assume that 1040 Line 15 ("taxable income") is the number I push into the tax bracket calculations.  Given that, is there a website anywhere that will show the tax calculations for a given taxable income input so I can try to figure out what's going on?  

 

Thanks!