Jack, I have had consistently short plants (shorter than the breeders specify in their ads.) I think the power of the Mars II is keeping the plants short. This has been working for me as I have so little height to grow in. If you have the space for taller plants I'd pull your light up to the 24"-36" distance that Top/Mars LED recommends. I'm going to try the smaller light I bought to see if I can get a little more height out of the 24 Carat this grow. It's also very cold now where I live so I think the problems I've had with this grow are due to more than the intensity of the light. I'll be by your thread to see how your light is working with the Northern Lights. Cheers!
BTW I am at day 56 and my tallest plant is about 10" tall. How is that silent extraction fan of yours working? Is it really quiet?