"where is the best seat placement on a 2010 seraph clone lite"
would need a lot more information about your situation. Check out Phantom Racing Chassis' web site or give them a call - they can help you out. Just realize that "best" will be a compromise, and you'll have to experiment to find your kart's best compromise.
"I have noticed on some karts the placement of the seat varies from center with the rotor to extreme left of the rotor as far as to almost cover the front of the lr tire. With the front of the seat centered on the steering support to just to the left of support. Is this due to achieving the correct percentages?"
Yep, and you'll also notice that karts set up for different weight classes will have their seats in different places. The driver is a significant piece of weight (adult drivers can weigh more than the whole kart) that can effect handling. Where and how you mount the seat depends on a lot of things - chassis design, overall kart / driver combined weight, track layout, and tires used are probably the main factors. For a left-turn oval people generally like the seat low and to the left side, but you might have to adjust so the driver can see and reach the pedals.
One word about "correct percentages". Those are guidelines, not absolutes. Every kart and driver combination is different, and the track can change too. Don't get hung up on a particular set of numbers. Find numbers from a trusted source to use as a starting place, but expect to experiment from there to optimize your own setup.
