"Exactly what does "figured without taking into account the amount from Schedule 1 (Form 1040),
line 20" mean here?"
It essentially means treating your traditional IRA contribution as nondeductible during the calculation of your MAGI. On your completed tax return, it would mean adding back the amount on line 20, exactly as you see TurboTax doing.
Unless your MAGI is clearly above of below the phase-out range for your filing status, yes, the MAGI needs to be determined to know how much of a traditional IRA contribution would be deductible. As I said previously, you can enter the maximum permissible traditional IRA contribution, let TurboTax calculate the amount that would be deductible, then go back and change your contribution entry to be just what is deductible. Alternatively, you can enter $1 of traditional IRA contribution to cause TurboTax to prepare the Traditional IRA Contribution Worksheet, look at the worksheet to see the MAGI that TurboTax calculated, then do your own calculation of the maximum deductible contribution, although that's a lot more work.
