You use the method used by snc76976 in his TAS video, but in a way that it will help you save 100% instead of 80% that snc saves in his video.
link for the video :
https://www.youtube.com/watch?v=KsQiAl8hWWkYou need to create the staircase snc creates but in a way that extends lower ( closer to the steel floor ). As I said , reproducing this staircase is easy with the Neolemmix mechanisms , but the solution will become tricky at the end, when the lemmings from the left hatch will reach the staircase and they will start going to the exit. In that case , to make your life easier do the following trick:
Use the three last lemmings to dig a wide pit that will have the steel floor as its lowest point, and the staircase as its highest point. Make it as wide as possible. When its finished assign one of these lemmings as a builder to build from the steel floor to the beginning of the staircase. In that case, the last lemmings will go to the exit too, and you will save 100% of the lemmings