Flucloxacillin isn't used in the US (it's banned for use in chickens), so I don't know dosing for it and have not been able to find a source of information for chickens. I would try calling your vet and asking if it would be ok to use until they can see her and what dose. That would be safest. Enrofloxacin is sometimes prescribed for chickens that are pets and not for food/eggs.
Common doses:
Amoxicillin dose is 57 mg per kg of body weight, twice a day for 5 days, orally.
Enrofloxacin dose is 10 mg per kg of body weight, twice a day for 5 days, orally.
Your medication is a combination, and I don't know what the percentages are, dosing may be very different.