Should I leave the basis for gain/loss blank on vehicle? Bought it 35k. Dealer trade-in 16k. Depreciation: $12,934. I have all the numbers in except for gain/loss for now

Yes, leave 'Basis for Gain/loss' and 'AMT basis for gain/loss' blank.

Once you enter Sales Price (trade in value), Cost and Prior Depreciation, TurboTax will calculate any Gain/Loss and report it on Form 4797 for you.

You can see the calculations on the Car and Truck Expense Worksheet when you download or print your return.