Let me see if I can make it a little clearer for you. Pete and Silvercreek have picked up some of the details already. First pick out 2 of the larger cdc feathers you have. Now look at the feathers, notice how the feather stems bend slightly downwards? Place the 2 feathers together (one on top of the other) with the bend side of both feathers facing downward. Now with your fingertips move the two feathers back and forth on top of each other a little bit until the tips of the two feathers are even. I don't trim the tips of the cdc feathers, I even them up by matching the tips up. Now pinch the two feathers and measure them against the hook shank. They should be just a little bit longer than the body. Turn a stonefly upside down sometime, you'll see the wings extend just past the body. Hope that helps
Any trimming of the cdc feather is done at the butt end of the feather.
*Extra little tip for times when you do need to trim cdc feather. I usually use my thumbnail and forefinger to pinch pieces of the feather off rather than trimming them with scissors, it helps to avoid that blunt ended look. It gives you some uneven ends that look a little more natural.