Diet can never be more than 50%. To be exact, when it comes to building muscle, you need good genetics (although anyone can improve on their shape with proper discipline and consistency), sound training principles, adequate dietary intake of all the necessary nutrients and a proper mindset. Diet plays a big role only when you are trying to achieve contest conditioning. Otherwise, as long as you take sufficient number of calories and macronutrients, especially protein, you will be OK - getting scientific and too exact about things like grams of protein per pound of body weight is a waste of time for the average person who is looking to put on some muscle and get a bit leaner.
Diet is never more than 50%. If you say that you won't build muscle without a proper diet, you are right, although you can't also build muscle with a great diet and lame training or piss poor genetics.