Make each increase gradually. Do not jump from one level to the next all at once. Fawns will always act like they want more. Overfeeding is the most common cause of diarrhea. A fawn's manure is normally soft but it should not be watery.
Prolonged diarrhea can cause death.
Fawns can actually eat themselves to death. You can use less powdered milk with the usual volume of water until the manure is normal again. Then gradually increase the milk powder back to the proper amount according to the recipe. If diarrhea continues, give a baby's dose of human medicine for diarrhea such as Kaopectate or Pepto-Bismol. Use the dosage directed by the instruction on the bottle.