I think it’s usually best if you can set a pace, or maintain a step rate, that will place you somewhere within the zone you’re training. It seems starting and stopping may not produce the desired result of continuous movement for many hours (or all day) that we’re training for.
For example, when I run, I set a pace that keeps me within the zone I’m training. If my HR begins to drop below the zone being trained, I speed up a little. If my heart rate begins to exceed the top of the zone, I slow down a little. Usually my HR will correct back into the desired range within several seconds of adjusting my pace. With practice you can stay in the correct zone for hours with little correction needed.
If the stair master speed can not be slowed enough to keep you in the desired zone, you may want to try another mechanism/machine (run/treadmill, bike, hike, etc.).