I use 150 gr core locks in my .308, and I shoot behind the shoulder like you, and I often hit a little higher than optimum, and I often have sparse blood trail. I don't worry about it, the deer always die quickly, and I always find them. If there is a situation, say a longer shot, heavy cover, precipitation, late in the day, huge buck, etc, where I'm really worried about not finding the deer, I'll hold a little differently. In a situation as I described, I'll take a shot that will break the deer down, such asshoulder, base of neck, etc.