Match fishing line test to lure weight.
Line test is not simply the weight of the fish. Two independent requirements set the floor and you take the higher. The first is the fight: a hooked fish generates peak loads well above its static weight through surges and head shakes, and if it can reach cover you need enough test to turn it before it wraps you around timber — hence a 2.5 times multiplier in heavy structure against 1.25 in open water. Braid needs a further allowance because it has no stretch to absorb shock loads that monofilament simply soaks up. The second requirement is casting the lure without the line snapping on the forward stroke, which sets a minimum for heavy lures.
Test for the fish and cover
Test lb = fish lb x cover factor x material factor
Final choice
Take the higher of the fish requirement and 60% of the lure-matched test; leader 20-50% above main line
Because you cannot let the fish run. In open water you play a fish out against the drag and light line works fine. In timber or thick weed you have to stop it immediately or it will wrap the line around structure and break off, so you need the test to apply real force straight away.
The leader takes all the abrasion — against teeth, gill plates, rocks and weed — and it is the section closest to the fish. Stepping it up 20 to 50 percent means that if anything does part, it is more likely to be at the leader knot than halfway up your expensive main line.