In a addition to Roy and Brent's information there is a bit of a process that I tried to recreate from memory that may be useful to you when setting the mast. My dealer taught it to me a LONG time ago:
http://www.hobiecat.com/community/viewt ... highlight=
After you complete that process of setting the main, then set the jib. The forestay will have a belly of 2 to 4 inches of slack. In other words the Jib halyard and luff cable become your forestay as Brent said. The reason you want this tight is for maximum upwind performance. You do not want the leading edge to fall off to leeward. Sailing physics dictate that you are not being "blown" upwind, you are being "pulled" upwind by an airfoil, therefore the better your airfoil (jib) is shaped the more lift you get.
Note, no matter how tight your sides are set, the leeward shroud will always get slack under load. Mainly because you and I can't exert nearly the amount of pressure that mother nature can.