I take it this is the form that treats drinking water that the whole flock drinks from. When medicating chickens by this method, you total the combined weight of the entire flock, prepare the watering container with the amount of water they all consume in one day, and add the medication following the dose per pound or kilogram of body weight.
It averages out that the small birds get the amount they need because they consume less than the larger birds and vice versa.
If the worming med is a paste, then dose each bird individually by giving the appropriate amount for their weight directly into the beaks.