I have had my cat (adopted stray) for 5 yrs. Every other year she gets fleas(inside cat), I put Frontline on her. The problem: Before I realize she has fleas, she gets this scab(pretty big) on her neck. I take her to the vet every time and they tell me she may be allergic to fleas, they give her a shot and give her a prescription of Amoxicillan. It ha s a dropper with ML on it, but the bottle instructions say to give her 10-20 Mg twice a day. Any experts on cats that could give me some advice would be great. I am broke right now and can't afford the vet and the SPCA is out of funds right now. My cat is roughly 5-7 yrs old and weighs roughly 8-10 lbs.

If you read the bottle it should say 50mg/1ml. I am saying this for future references only. Right now the antibiotic, (amoxi) was rx's as a preventive to combat infection from her scratching simply put she cants clean it herself unless she has some very long tung. So for every full dropper, 1ml is 50 mg. So roughly 1/2 the dropper full is close enough to obtain 25mg of med. Make since? I have a fat lab that is also allergic to fleas, an requires advantage more often that the recommended 30 day intervals. Advantage is very safe, can not cause an over dose an works great! In your case as well as mine with a pet that has developed an allergy to fleas, just takes a little more aggressive treatment to keep them comfortable.
Also I have posted several other aspects of flea meds just a day or so ago in the Dog section. Check them out, good info many people don't know about fleas, sorry I am too lazy to type them over again.
Good Luck!