If you are using the poultry vitamin by Rooster Booster, the mixing instructions are on the label, and it is the same for all ages since young chicks will drink less. If you are using B complex tablets, then 1 tablet per gallon should be good. Some use PolyVisol without Iron baby vitamins for chicks, and the dosage is 2-3 drops daily by mouth.
It is sad that she injured the leg. I would definitely use a poultry vitamin, and let her rest the leg, and hopefully she will start to use it when it is more comfortable. You can tape the toes or use chick shoes to straighten her feet, but there could be nerve damage. Here are some good links to read about taping toes and making chick shoes:
