Ok thanks for clarifying, but does it always, by definition, have to be the case that my focus point at night with a 0.8x reducer will require racking in the focuser farther than I do when I'm in focus at night with just the flattener? In other words, if I'm in focus at night at step 85,000 on my focuser when I am using the flattener, can I declare with confidence that achieving focus at night with the 0.8x reducer will require me to place the focuser at some step position that is less than 85,000?
Not exactly. The focuser position is a different thing than your actual focus position. A little explanation is in order.
First, a refractor has a fixed focal plane position behind the objective lens when viewing objects at infinity. As you bring objects closer, that focal plane position will move farther back, away from the scope. Hence the day/night difference.
When imaging, without any reducer or flattener, the sensor must be at that same focal plane position to be in focus. If you add a reducer in the image chain, then the focal plane position moves closer to the scope. This is always true.
But your focuser position is affected by other things that may be in the image path. For example, lets say that without a reducer, your focal plane is 100mm behind the fully-racked-in focuser. If you have a 50mm long extension tube between the focuser drawtube and the sensor, then your focuser would have to racked out by 50mm for the sensor to be at the focal plane. If you have a 40mm long extension tube, then the focuser would have to be racked out by 60mm.
In this example, the focal plane position has not changed, but the focuser position has. Point is, the focuser position is NOT an indication of where the focal plane is, or whether it has changed.
So, back to your situation. You have two different devices in the image chain, a flattener, and a reducer. I'm not familiar with those two, but I would assume they are not the exact same length. So they will change the focuser position in the same way as my previous example, regardless of whether they change the focal point or not.
Bottom line, measure the distance from the rear of the scope (not focuser) to the sensor when you're in focus. That distance should get shorter when you use a reducer.