Tilt the seat so that it supports your bottom and your thighs evenly. You don't want pressure points at the end of the seat. Make sure your thighs extend passed the seat so that the back of your knees do not make contact if at all possible.
Your legs and pelvis should have ample ability to move and shift position without detracting from your driving. This will relieve pressure points and keep blood circulating during long drives. Staying in a cramped position for too long may lead to Deep Vein Thrombosis.