If she's drinking I think that would be sufficient. I mix it stronger for babies and the ill.
Have you checked her over for any injuries? You say she can walk but it's like it fatigue her quickly, right? And she's coughing but does her breathing seems labored? Sneezing? Nasal discharge...
Here is my Holland white. She's three months old (I hatched her myself). She might help you narrow down your's age. The blue is also three months and the red is a week or so younger.
Even a pet turkey needs the higher protein. Does she free range? Has her head changed color? Could the heat be getting to her?
In any case, she shouldn't eating layer feed.
My Holland white is 3 months old. I'll post a pic in a bit too help with age.