OK I figured it out myself. There's paperwork from the broker that shows the adjusted cost basis, and you have to use that amount for SPP stock sales. The "Adjustment Amount (Ordinary Income)" from the broker form is the same thing as "Compensation Income" in Turbo Tax. That value is the adjusted cost basis minus the cost basis on 1099-B. That value may also appear on your company website under "W-2 breakdown" for example.
