Taking any general amount off (historically) like a third or whatever system you use is nonsense, every pup is different, some will be born with tails that barely need anything off and some will want more. When I take my pups to the vets I insist on showing him where to dock, no way I'd leave it down to him. I hold each pup up and judge the tail as if it was a mature full size terrier, the shape and size of the tail never changes through out it's development, if it looks right as a babe it'll look right when it's mature.