Are you going to be using the LPC NG or the QRH? The QRH never has the 15% margin added to the landing distance, however the LPC NG if configured by your administrator will automatically add the 15% to the landing distance (both with and without failures)
You should also look over the QRH for the runway assessment matrix as this will guide you what values to enter either the tables or the LPC with... Note that you select different options for dispatch and inflight calculations.