Bike saddle position for mountain biking/jumping?
The fore and aft position of the saddle is a highly personal position. You're going to have to experiment with it to find where the saddle should be for your style of riding, terrain, your proportions and your bike. Peter White has an article on bike fitting, which may be of interest to you. The most relevant passage is: 


  take the case of two riders; Rider A, and Rider B. Rider A has very
  little upper body muscle but very strong legs. Rider B is identical to
  Rider A but has been working out at Ralph's Gym and looks like a body
  builder. The fore aft position of the saddle will be slightly
  different for the two riders. The extra upper body mass of Rider B
  will require a slightly further back saddle position to give the same
  balance. But that doesn't necessarily mean that Rider B should have
  his saddle further back. He may prefer the more forward position. Only
  he knows what his preference is.


Note that for mountain biking, you also have the option of moving the saddle up and down if you install a dropper post, which can also be useful depending on your riding style, terrain, your proportions and your bike.