Lens shift adds another variable to placement, albeit and beneficial one. The HT3550 has a lens shift of +/- 5%. This directly impacts the offset of the image above. There is a dial on the projector that can adjust the offset +/- 5% up or down.
https://www.amazon.com/BenQ-Entertainment-Projector-TK850-Watchers/dp/B07Y2N2MBL

