Want to calculate rental expenses myself, rather than have TurboTax allocate. (I own and live in triplex.) Can't go back and change this choice.

Yes.  You can choose to make the entries yourself especially since you know exactly what belongs to the rental.  Sometimes it's easier when you already have the figures you want.

Do not select that the property was converted to business use. (see situation screen image attached)

Be sure to select "yes" it was rented all year and that it was always rented at fair rental value.

Continue to enter all the actual income and expenses that applies to your rental property.

When you enter the house as an asset you can use as the cost basis the lower of your actual cost or fair market value (FMV) on the date you converted it.  FMV is not the appraised value but rather what it would sell for on that date.

During the asset entry you will be asked the date purchased (date placed in service) and you will enter the date you converted it to a rental property.  This will apply the proper amount of depreciation.