If you look at your 2019 MI-1040 tax return, you will take your total Michigan tax which is shown on Line 21. From that number, you would subtract any refundable credits on your 2019 return, which are shown on Lines 25, 26, 27b and 28. I have attached a picture below for additional guidance. Your total is the amount you would enter into TurboTax to calculate whether you should be assessed an underpayment penalty for 2020.
If you filed your 2019 taxes with TurboTax, sign in and go down to Your tax returns & documents and you should be able to view your prior year Michigan tax return.