I know of several people who have used their own amoxycillin prescriptions for their chickens. As long as you dose at the rate of 10 mg pure amoxycillin per kg weight of your chicken, it all works the same, no matter who it was originally prescribed for! A standard production red chicken weighs around 2 - 2.5 kg, whilst larger breeds like Sussex and Marans can weigh 4 kg or more. You therefore need to get 20 - 40 mg of amoxycillin into your chicken each day for 7 - 10 days. The best way is to measure out enough for say 4 days, sprinkle it onto a small amount of a treat like boiled white rice, and then divide it into 4 portions and give her one portion each day. It's not terribly exact, but it's usually accurate enough to give good results (at least it has been for me so far!)
All the best for your girl - let us know how she gets on.