Basically in a nut shell this is the procedure i am currently doing:
Found the center of the crafts length and made a mark that was 50mm behind this line towards the duct end I then placed a pole under the craft at the 50mm mark i made and then placed the motor in the rear of the craft. My wife and I sat in the drivers and passengers seating position and then shifted the motor back and forth until the craft become more tail heavy than anything else. Kept shifting the motor until it got to the point where if i (the driver) was to lean forward a little you could just feel the back of the craft wanting to lift. I made it tail heavy so that when thrust is applied it wont try to plough in everywhere i go.
I'm thinking that having it adjusted to suit myself and my wife's weight is going to be more appropriate seeing as that's who most commonly will be in it, and if i am in it on my own then i will add some weight under the seat to act as the weight she would otherwise be providing.