What is the ideal seatpost length for a mountain bike, and how does it vary depending on factors such as rider height, inseam, and type of riding? For example, does a longer seatpost provide more power and efficiency on climbs, or does it compromise handling and stability on technical descents? Conversely, does a shorter seatpost improve maneuverability and reduce the risk of over-the-handlebars crashes, or does it lead to reduced power output and increased fatigue? Are there any general guidelines or formulas for determining the optimal seatpost length, or is it primarily a matter of personal preference and experimentation? How do manufacturers and bike fitters typically approach this aspect of bike setup, and what are some common mistakes to avoid when selecting a seatpost length?