While the oft-repeated axiom is the length of the ear when laid back against the neck, bridle path length really depends upon the horse and what flatters that individual's neck. In my opinion, however, half the mane NEVER looks good on any horse...reminds me of an almost bald man with a pony tail. I would take it back bit by bit until you find a flattering length.
Another factor is neck carriage. To my eye, an upheaded horse whose mane falls back and clear of the throat latch/neck can handle more mane, less bridle path.