Amme, 40 lbs sounds good. But what really matter is how does he look. not too skinny, not too fat. if so they are good to go. And as already pointed out here, you never want them to grow too fast. You sometimes hear that, a dog is small for its age. Well they will be as big as their genetic makeup has destined, and for the giants the slower they get there the better. I agree to a certain extent with taijinrrr, that you can not only look at the paws, but they definately do play a roll in the size, the safest is to look at a lot of tings, the paws, the growth plates, the bone structure as a whole.
With a large dog you really dont want them to have more than 23% protein, ever in their food, that will make them grow too fast and put stress on their joints. One very common thing you can see when they grow too fast, it that they knockle over a bit, and that the inner bone at the front knees grow a bit faster than the outer bone, it can be sign of too little calcium but most likely just a growth thing. Mind you the real big guys can knockle over even if they grow as slow as you can make them.
Danes tend to fill out way later than other giant breeds, one female I once had went under then name "the picket fence" all legs and very little body lol..... but she turned out real nicely. Last time we weighed Galahad he was about 14 weeks maybe a couple days more, and he was 58,2 lbs, so the mastiffs are a bit heavier than danes. And grow different, the body tend to actually grow with the legs haha
Marit