The starting point is always to consider which type of fly you would need for the water you are fishing. When you know which size of fly you are going to put on the leader, then you will also have an idea of the line weight needed. For example, a large and heavy fly takes a heavier line and a strong front tapering (which equals a higher line weight), in order to be able to cast and present the fly properly. The following table is a list of different line weights and what I believe are the fly sizes that match those line weights for trout- and salmon fishing.
The weight listed below is the loading weight for any of our line systems. The weights listed are the actual weight for a shooting head, or the weight of a belly if it is a weight forward line. Do not count or include the leader’s actual weight as part of the loading weight. The leader has a completely different function in a cast. It is a mass (air and weight resistance) that balances the speed between the upper line and the lower line when the loop is traveling forward. It is not part of the line weight that loads the rod and should therefore not be calculated as line weight.
If I had to go into more detail here, this recommendation would turn into a long and detailed article, so please just take my word for it.
All our leaders are measured to have the correct mass for each line weight. Using Logic coated leaders gives you the practical advantage of allowing you to have more control over which water column your fly will be fishing.
The recommendation of fly size versus leader length when using Logic nylon leaders can be found under the Logic nylon leaders page on this website.