Simply enter the discount percent your plan uses. That percentage can be important if you have a "qualified" sale since the calculation of the compensation created by the sale can be applied to your grant date per share "fair market value" in determining compensation in that case. If you don't have a qualified sale that percentage is ignored, but until you enter the Form 3922 information, TurboTax doesn't know if you have a qualified sale, or not.
Tom Young