I just recieved a CP11 notice. How do I go back into turbo tax and double check how the program came up with my results for the recovery rebate credit?

It's pretty straightforward.  How much did you get last year?  Turbo Tax calculates how much you are eligible for.  Usually 1400 for each person.  Then subtracts what you already got.  Line 30 is only if you didn't get the full amount or qualify for more. 

How much is missing?  

Did you get IRS letter 6475 saying what they sent you?  If you didn't get the IRS letter 6475 saying what they sent you then check your bank statements and check your IRS account to see when and what they sent you.

Your Online Account | Internal Revenue Service

If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment

https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-payment-issued-but-lost-stolen-destroyed-or-not-receivedYour Online Account | Internal Revenue Service