First off your leader strength needs to be judged against the weight of lead you are casting. A safe guide is to factor by ten. For 2oz use a 20lb leader, for 4oz leads a 40lb leader, and for 5oz a 50lb leader. Really powerful casters with good technique tend to use 60lb for general 5 to 6oz fishing.
When power casting the leader knot joining the leader to the main line needs to be off the spool and fully through the rod rings just before the spool of the reel hits full speed. If its not, then the knot will catch in the lower butt or intermediate rings and cause a crack-off.
The best way to measure the required length of leader for casting over cleanish ground is to tie the main line to the leader material outside the top tip ring, wind on the leader counting it around the reel spool 8 times, then cutting the leader off beyond the tip ring leaving about 36-inches hanging free for simple casting styles like overhead thumps. This gives you a good drop using average length rigs of around 30-inches, and keeps plenty of line wrapped around the spool for casting strength. You need the leader wrapped around the spool a minimum of 8 times to give the strength when casting to initially pressurise only the leader and still have some in reserve on the spool until the spool hits full momentum to protect the weak reel line underneath from early stress.
For casting with half and full pendulum swings using anything up to 10-feet of drop from the rod tip to the lead, then cut the leader leaving 6 to 8-feet of line off the end of the rod tip depending on the type of rigs you’ll be using and their average length. Adding the rig to this gives you the full drop to suit your casting style, but also gives plenty of turns around the spool to protect the main line, again from initial casting stress.
On occasions, say off rock ledges, piers or breakwaters, it can pay to use a much longer leader, say between 30 and 40-feet to ensure that once a decent fish is brought below you, you have the strength of line to handline the fish upwards if need be. In these cases though, power casting is rarely used, so there is less chance of the leader knot fouling on its travel through the rod rings as the overall speed of line during the cast is less. A more subdued cast is the key with leaders longer than 30-feet.
You can see from this that it pays to use a different leader for different types of casting and fishing. This means a new leader for every trip judged against the fishing situation you’re tackling. Using a new leader each time also guarantees you have no inbuilt weakness in the leader left over from the previous trip that might cause a crack-off.