As mentioned above, the uncut spring length should be correct for your setup. 14" past the end of the extension would be ideal, but from much of the feedback I get from customers, they'll run fine a little on the shorter end. Do not simply drop the spring in and go, however. Your spring has unclosed ends that can cause problems when loading and firing, and if not tuned, will shorten the life of your spring drastically, or break it immediately.
Grab the last 1/4" or so of the spring. Turn the sharp point in and down, into the coil. Repeat on the other end and you should be good to go! Your spring should last a good long while, but as a wear part, we recommend you replace it once a season or so.
-Team Nordic