I’m not sure what thread you read about sensor placement, but if you are approaching the sensor straight on, it will have the shortest detection range. That’s because these PIR sensors detect very small values in heat passing across the sensor field. They can often be quite blind to something moving straight towards them. This is one of the reasons that people put them on the ceiling pointing down, then the person is crossing the field underneath.
The following FAQ might help (this is a clickable link)
Your landing case is a very difficult one when you only have one sensor as half of it will always be straight on unless, again, you put the sensor on the ceiling. And then it probably won’t catch people right at the beginning of the stairs going either direction, which won’t be what you want. It’s more likely that you will need one sensor at each end of the stairway.
But read the FAQ and see if it helps.