OK, start with the stock settings; see how it feels. Moving the seat *up* will increase the side moment in a turn, and increase grip. But be careful - lots of grip going through a chicane can result in squirrelly oscillations and loss of control. Some guys do raise the seat in the rain. Lowering the seat reduces grip but makes the kart a bit more stable.
Moving the seat affects grip balance (and center of gravity). Move the seat forward and there is more weight on front tires and more grip. Too much and you will be fighting oversteer, especially coming out of corners if you don't 'square up' the kart before getting on gas. Push the seat back and you will be more stable but now you may understeer or 'mush' on entry. This is really a personal preference thing based on how you drive. A lot of fast guys started out with the seat between -.009 and -.006 and slowly came forward to -.004 to -.001. That makes the kart very responsive, but may be difficult to handle at first. Get to a spot you like, and when you feel confident try going forward .001 at a time.