To calculate the adjusted cost basis using your form 3922, you will use a "Range of Dates" when you got an option to buy the espp stocks.
For example the 3922 will have a range between January 2nd and march 31st between column 1 and column 2. This means that you had the option to buy ESPP stocks during that time, that same row will contain the values of the stock at different times during that range of dates, things like price at grant date or price at exercise date.
If you want to know during which period you made the purchase (hence which row's data to input in turbo tax software to calculate the cost basis for a stock sell) you will have to match the 1099-B "Date of Aquisition" date to the date range of the 3922 form.
For example if you have a Date of Aquisition in your 1099-B of 04/23/2019, you would use the row with Date option granted = 04/01/2019 and Date option exercised = 06/30/2019 in your 3922 form. This is because April/23/2019 is between April/01/2019 and June/30/2019
NOTE1: The form 3922 might need to be from a previous year and not form current year. This happens when you sell shares that were acquired years ago but sold this year.
NOTE 2: The Adjusted cost basis value in the supplemental information of your 1099-B should already consider these calculations so there is no need for you to do them, they should be accurate.
