IMHO , the leader should be longer than the fish you intend to catch but considerations for longer leader is to enable easy tracing the fish so it can be easily released , extra long leader is necessary when one cast from the bow of the boat , any 40+ ft boat will be some 5' above the surface of the water , add the waist height and some , you are likely to get a minimum of 9' before adding cut offs . If one fish from a small boat , extra long leader isn't necessary and either mono or PE line on the index finger is personal choise .
What affect casting distance are a few ; too bulky a knot between PE n mono leader , too small diameters of the guides including the tip top , too big diameter of the PE n mono leader are among those which will reduce casting distance .
My preference are PE4 - 80 lbs. mono leader , PE5 - 100 lbs. mono leader , PE6 - 130 lbs. mono leader , PE7 - 150 lbs. mono leader and PE8 - 200 lbs. mono leader . I may add 6~8" of 400 lbs. fluorocarbon bite leader crimped on both side , when deemed necessary .
Jon .