A simple formula we use in warmwater is 60% of the desired leader lenghth is the butt section ,20% of the total leader lenght is tippet and the remaing 20% is the middle piece that fits bettween the butt and the tippet. On a 4 or 5 wt rod we start out with a leader butt of 20 lb. For trout I generally buy a tapered leader, and add tippet or possible a middle piece and tippet as the season goes on.