It depends on background cover, tree structure and shooting lanes. If there is good cover and good lanes at 12', then 12' is high enough. If it is December (no leaves) and the deer are skiddish, then 20' would be a minimum.

Remember that double-lunging a deer becomes exceedingly difficult if the downward shot angle is too great. If you are 25' in the air and the deer is at 7 yards, then your target area for a lethal and humane shot is very small and surrounded by bone - even if the deer is perfectly broadside. We talk about increased hunting height for concealment but we often neglect to consider the effect on shooting angle.