Biathlonman,let bears come in to the bait and feed for a bit and settle down.They will be cautious for a bit till they settle in.
I wouldn't shoot when they are on their belly feeding.In that position the shoulder blade can roll down,almost horizontal and block a good bit of the lung.For sure,I would hold half way up as far as top to bottom.Mature boars have a pec muscle that hangs down quite a bit below the sternum.Like Pete says,the sternum slopes up but the pec muscle looks like the bottom of the chest.If you could see a bear like that from the front,there is a deep cleft between the pecs.
Double lung a bear with any sharp broadhead and he will go down very fast.Good luck on your hunt and have fun.