Milligrams. You would use twice as many drops because your ivermectin is half the concentration. As for frequency, I see anything from 1-2 weeks in between.
0.2 mg/kg and 0.5% is 5 mg/ml so 0.12 ml for a 3 kg bird. It's very non-toxic to most mammals and birds so precise dosing isn't necessary. Instructions for 1% ivermectin say to apply 1 drop per 500 g of body weight so you would use two drops per 500 g.
Well, ivermectin and moxidectin are the drugs you want but they're basically water insoluble. There are some versions that use an emulsifier or solubilizer so it can be put in drinking water but they're marketed for pet birds and may be too expensive to use for chickens. S76 or Moxidectin Plus...