Hi,
I use NOROMECTIN and the recommended dosage is 1ml per 33kg of body weight, I was advised to use this on the pigs as it covers all internal and external parasites.
The Vet tells me to worm the pigs at 9 to 10 weeks SO that there is a small margin for error.
The way I do it is to weigh the heaviest and the lightest and take an average.(This is where the margin for error comes in!).
I suppose if you have the time you could weigh each and every pig, Depending on how many you have!.
Hope this helps
Kindest regards
Joe