A wind knot often takes the form of an overhand knot. An overhand knot can reduce the strength of the leader section where it ends up by 50% because the knot literally "chokes" itself. However that does not mean that it reduces the strengh of your leader by 50%. Why? Let's say the knot falls in a section of leader that is 10# test and your tippet is 3# test. The 10# section got reduced to 5# by the knot, but it's still stronger than your tippet.