Thanks Dans,
I don't know yet what drench I will be giving them. The vet said they are able to give out small doses rather than me having to buy a whole bottle. It just depends on what they have open when I go in next week, but I will take into account what you have said if it contains closantel.
I am reasonably happy with the calculations I have but we are considering getting a spring balance. I can't believe how cheap they are. OH says he would be able to rig up a pulley and I guess I could use an old sheet for the sling.