The Universal Joint should be placed as close as possible to the actual base plate of the seat. You also need to balance the rig to determine where it bolts to the seat base, find out how to do so here: http://www.xsimulator.net/community/faq/find-the-center-of-gravity-to-balance-a-motion-simulator.55/
As @SeatTime explains the proper placement of the Universal joint best simulates a race car, as detailed in this image: