How can I use turbotax to do an end of year tax estimate?

You need to use the 2016 program to make 2017 estimated tax calculations... but since the 2016 online version is closed you could use the 2017 program HOWEVER if you use the online version you will need to be very careful to undo all your "what if" entries when you complete the real tax return.

It is always best to use the downloaded version for "test" or "what if " scenarios ...

If you want to play around with different figures and tax scenarios without affecting your original return you can

>>>In the TurboTax CD/Download software by creating a test copy:

1.  Open your return in TurboTax.

2.  From the File menu, choose Save As.

3.  Give the copy a new name to distinguish it from the original (for example, by adding "Test" or "Example" to the file name).

4. Click  Save. You are now safely working in the test copy and anything you do here will not affect the original.

 https://ttlc.intuit.com/questions/1900642-how-to-make-a-test-copy-of-your-return

>>  use the WHAT IF tool:

- Click Forms Icon (upper right of screen) or Ctrl 2 (forms view)

- Click on the Open Form Icon

- In the “Type a form name.” area type What-If (with the dash), click on the name of the worksheet - click on Open Form

- You will see the worksheet on the right side of the screen; enter the information right into the form

- To get back to interview mode - click on the Step-by-Step Icon (upper right of screen) or Ctrl 1