So the egg was intact, but just broken in the nest?
If she's acting normal, doesn't have a discharge from the vent that looks like albumen (egg whites) or egg yolk, then I would discontinue the Amoxicillin.
You may want to go ahead and give her extra Calcium for a few days though.
For future reference, you ask the dose of Amoxicillin. Standard dose is 57mg per pound of weight given orally twice a day for 10-14 days.
So a 4lb hen would get 228 mg twice a day. If you have 500mg tabs, then I would just give her 1/2 tab (250mg) twice a day for ease of dosing.