My aunt had a 120 lb lab named Bo and his tail was a WMD!!! When she would come in town we had to clear everything off tables, counters etc.. She ended up having to dock his tail for the same reason. He was such a happy excitable dog that he would hit it on the wall or their fence and split the poor thing. Oh, it was miserable. once they docked it they never had a problem. Sometimes Bo would look behind him, i think it was "phantom tail", lick something that wasn't there and then walk away
So, docking a bull mastiff's tail would be really sad
but if its what your girl needs, thats what she needs.