Thanks for writing in with your questions. Lots of folks are forced to use stair machines for training so it warrants some discussion.
I’m not a StairMaster master. I’m lucky to live where I have easy access to plenty of steep terrain. So, I might be a bit off here as I have only used a stair machine a few times.
I think you need to set the resistance on the machine to reflect the total weight (you and the pack weight). Increasing the “weight” setting will slow the machine down and decreasing will speed it up.
From the feedback we get from our coached clients who use these machines a lot it seems like one the machine the pack weight needs to be higher to induce the same sort of local leg muscle fatigue when using a steep hill outside. I can only guess that is because you do not raise your center of mass the full height of the step on the machine. Instead the step falls away from under you such that you only raise you center of mass about 1/2 the actual step height. While this might reduce the effect a bit form out doors we still see great gains among the folks using machines like this.
If you were fresh and rested then I think and felt that leg fatigue below 178 then I would say you probably nailed the ME training effect in that workout.