I have a Petrel,a little sister to the NH,I believe.
I put a skeg in at around 25% OAL forward from the tip of the stern.I don't think you need to be absolute with this position.I had to make a better skeg with pivot bosses as I found Sand did jam it up and since then there have been no issues with deployment and is a great addition to use when the hull is not engaged in the water enough due to my young daughter's light weight.I use the skeg allot for down wind paddling and think it a great way of changing the boat's yawing characteristics.
I think off setting it from the center line to avoid sand jamming is a little too hopeful.The sand is everywhere when beach surf launching.
Have a look at how I did it here.The control lines work well but I find gradual deployment is hit and miss.All or nothing, mainly, I suspect.
http://www.kayakforum.com/cgi-bin/Building/index.cgi/page/1/md/read/id/222434/sbj/strip-a-skeg-story-and-a-good-solution-maybe/
cheers,