A general rule of thumb when selecting the proper tippet (for trout) is as follows. Take the hook size you are fishing, Let's say a #12 caddis, and divide the hook size # number by 3 or 4 to figure out what X tippet you need. For example, if I am fishing slow water I will divide 12 by 3 and go with 4X. If I am fishing faster water I will divide it by 4 and fish with 3X. Just a quick rule of thumb, but it might help be helpful.