My twin weighed 7-5 and 7-4. I don't really focus on weight gain during pg, so I'm not entirely sure, but I measured the same as I did with my singleton until 30 weeks (one of the reasons I didn't know I was carrying twins). I guess I must have weighed around the same as well? In the end, I gained a total of 55 lbs. I just looked that up on my dr. records.
I never read Dr. Luke's book but from what I've heard it has sound advice. I think if you eat healthy, focus on taking in a lot of water and protein, use quality supplements (RRL tea, prenatals, fish oil, etc.) you will do fine. Try not to worry about the actual weight gain. Best to you!
ETA: Mine were born at 39wks +5days. And now that I read the other responses, I have to say that I can't see how McDonald's would be good for anyone, least of all a pg mama! An occassional indulgence if that's what you like? Fine, but to advocate it as healthy? I can't agree with that.