I've tried using the recommended seat positions on several types of Boeing, using exactly the procedures in the manuals and have always ended up in a posture where flying is almost impossible.
I have found that if a) I have a line of sight down the coaming and over the nose, b) the instruments are visible and c) I can get full control movement without hitting myself, then I have a good chance of taking off and landing without too many issues. Observing people I fly with, there is much variation in how they sit, so there's probably quite a large optimum band.