If she is not drinking well, then you can give her some of the water/corid mixture orally several times a day. It's important to get the meds in her, if she's not drinking then she won't get it. Once she's feeling better she will start drinking on her own.
Corid is really safe, if you give a bit too much it's very unlikely to do any harm. Since it's .1ml per pound of weight, I would just give it your best estimate. I'm not sure how much a 4 mo. old cornish would weigh, I don't raise them. But as said, it's really safe so I wouldn't worry too...
Liquid Corid 9.6% dose is 2 teaspoons (or 9.5ml) per gallon of water.
Powder Corid 20% dose is 1.5 teaspoons per gallon of water.
Treat for 5 -7 days, only water they have access to.
You can also give the liquid in an oral dose, to help get the meds in them quicker. I would do that with any...