If during the questionnaire, the "question" does not come up automatically (which it should be, however, there was an update this past weekend...) TRY THIS:
Click on the "PERSONAL" tab at the top > then click on the "DEDUCTIONS and CREDITS" section just below it > select "I'll choose what I want to work on" > scroll down to the "Charitable Donations" section and select "Donations to Charities" at the far right > then select "Cash Donations" and enter $300
Now, if it hadn't brought up the question at the end and you choose to take this route (it's really fast), you still might get an "alert" to enter the $300 again - this is the 100% line that requires that $300 to get carried to. I just tried this after the update and this is what I had to do to get it. I hope this helps! I will let PS know of the problem.
