was able to get it calculated right today and submitted/accepted. the issue really is that the input screen in TurboTax is missleading. When you get to that screen enter your data (Make, VIN, purchase date and service date ...). In the box at the bottom put 7,500. It will then fill out the form 8936 correctly (generate the pdf file and check) - that is it will put 7500 in line 4a, it will calculate the phase-out percentage (50% or 25%) in line 4b based on your other input and it will show the correct tax credit amount in line 4c.
Seems there's still one issue: The phase-out percentage should be calculated based on the 'service' date, but TurboTax actually does based on the purchase date.
