RE: how low to aim
I would think that aiming at the bottom of the body and behind the front leg would be low enough. Though deer can duck under about a foot. I had a doe jump the string last year.I was aiming fairly low on her and I shot about a foot over her back.
The best advice I can give you is to try your hardest not to spook the deer. They're way more apt to jump the string if they're alerted.