It's easier to under-do it than to over do it, really. Ivermectin has a wide safety margin.
If you're using ivermectin, not eprinomectin, then you'd use something like this:
1 drop small bantam - micro bantam sized
2 drops regular bantam hen
3 drops regular bantam male
4 drops small standard hen
5 drops standard chickens
6 drops large standard
7 drops giant breeds
I use drops that I make from a 3 cc syringe with a 25 gauge needle just because I find it much easier to make one drop at a time with that than a regular eye-dropper. (And of course it's not injected nor is it given orally). And of course aim for the lower back of the neck or some naked bit of skin on the main body, not the combs etc.
You don't have to repeat it but if you do, do so in 6 weeks. I try not to use it often in case of resistance - twice a year usually.
From my friend, MrsChicken1954: cider vinegar
If it's cocci you have to act fast - you have hours not days. It's no good using a weak dilution as you have to hit the coccidia fast.
The doseage I used was a complete guess but turned out to be effective. For a battery-sized chicken (or a full grown large fowl silkie) you'd use 3-5ml of vingegar and for a 6 week old normal sized chicken (or half grown silkie), 2 or 3 ml. But although it's unpleasant for them to have it, it will save them.
Take a syringe and put the amount of vinegar you are going to use, plus a little extra to allow for spillage and spit-age. Only dilute it a maximum of two times. So, if you have 3 ml of vinegar, not more than 6 ml of water. If she is at death's door, forget the water and just use the vinegar, then put a little water down her beak afterwards - she will be too shocked to drink voluntarily. My husband has tried a vinegar mouth-rinse and he went a very strange colour - it is horrible.
You will have to hold the chicken still and she will not want to let you put this down her beak. She will cough and gasp and it may leave her with the sneezes for 24 hours or so afterwards if she inhales some, but it will not do her any harm if you don't actually drown her. Anyway, give it at night and again in the morning and again at night until she is eating and behaving normally.
The vinegar I used was 5% acidity - it says so on the bottle. If yours is less acid, then put less water with it. It is the acid which hits the cocci. Good luck - let me know how it goes.
Tylan 50- injectible dosage..... 1/2 cc for bantam size until cleared up (usually 3 days) into the breast muscle.
Never again will I try to diagnose my own flock's ailments. I sweated for weeks thinking I had Coryza, after a necropsy I discoved it was a mycoplasma, treated with Tylan. By the way, the Tylan did the trick!