Rather than 40% I think the technical way to think about it is to multiply the unfactored landing distance by 1.67 and that will yield the regulatory requirement distance. The 115% is a turbojet factor for wet runways. I cant remember the regulation for that. So you get the base number or unfactored landing dist. Thats what a jet part 91 can use. Then the FAA says as 121 we need to land within 60% of the useable length (the whole obstacle plane/50ft phrase; displaced thresholds) so we multiply the unfactored distance by 1.67 and that number gives us a total landing distance that assures we will only use 60% of the useable runway. Further, if the rwy is wet, as a turbojet, we are required to add 15% to the landing distance. This is where the landing cards do allthe math for you. Its gonna be nice when the release the app for landing calculations. Think TPS for landing for your iPad.