When it's on the chest, my elbows are stacked directly underneath the barbell. This is an efficient stacked position for the bottom of your bench press. It allows to push the barbell up and back at the same time.
As you’re driving the bar up to the locked out position, many will have a tendency to become “unglued” and let their elbows flare out immediately. Instead, think about keeping the elbows tucked and underneath you when driving the bar back up.
During the descent, you want to drop your elbows down at a 45 degree angle relative to your torso (when viewed from the top) and... when viewed from the side on the elbows shouldn't be too far out in front of the bar - and you can make it a goal to keep the elbows stacked under the bar. When viewed from the side, the descending bar path should be down and slightly forward at the same time.
If your elbows are flared on the way down, that's gonna stick a lot of pressure on your humerus and you're looking at a shoulder injury. If your elbows have no place to go when you touch the bar to your chest, then flaring the elbows is what gets horsepower back under the bar once you get about halfway to 3/4 of the way through the bench press. If your elbows are already flared, they have no place to go... it's just gonna stop and fall back down to your chest. So what I recommend doing is trying to keep your elbows tucked at about a 45 degree angle. You do not want them right next to your side... and you do not want them flared out.
Upon analyzing injuries during flat bench press, Green and Comfort (2007) explained how shoulder abduction at 45 degrees with a medium grip offered the safest method of bench press performance for the shoulder joint.
As you’re pulling the bar down, think about tucking your elbows into your sides. This will not only help you in pulling the bar down, but it will keep your more stable and reduce the stress on your shoulders as well.
The ideal amount of adduction produces a vertical forearm when viewing the lifter from the front and profile views. This represents the most efficient way to transfer force from the shoulder girdle, through the arms, and to the barbell.
When the bar's in line with your wrist joints and forearms, then what you have is what us powerlifters call a straight line. And the strongest and shortest distance to move a heavy weight is through a straight line. Keeping the bar in line with your wrists and forearms will also save you undue stress to your wrists. If the bar rolls back in your hands, it's going to hurt like hell.