I'm looking into purchasing a long lead for my dog aggressive rescue dog. I'd like to use it for recall training, but also to give him a little more freedom to run around in open spaces when we are out on a walk. His recall is pretty good but disappears entirely at the sight of another dog. Given his needs, is a 10m lead sufficient, or should I choose a 15m lead? He is a GSD/border collie cross, around 22kg, so certainly not small, but not enormous.