Could just be the angle of the photo,
But I use the center of the gas tube channel, and the center of the front take down lug.
As for the real test, pull the upper receiver off the lower, then with it upside down, insert the B/C in and see if the center of the bolt lugs are indexing center wise with the receiver extension lugs.
Next to weed out a chamber problem alone (such as not getting it correctly clean, or a problem with reloaded ammo), without the B/C, muzzle straight down, drop a round into the chamber, use a wood dowel, tap the back of the case to make sure that the shoulders of the round are fully mated against the shoulder in the chamber, then turn the muzzle straight up. The live round should fall right out.
If a factory loaded live round does not fall out of the chamber (and you have been using a chamber brush to clean the chamber with CLP, then using a sharpie pen, and smoke the bullet of the round. Now do the test again, and this time after you use a cleaning rod to tap out the live round from the chamber, look at the bullet for any signs(scrap marks on the smoked section of the bullet) of embedding into the rifling at loading.
If the problem is with reloaded ammo, then first use an ammo test gauge. This will tell you if the cases where not correctly trimmed, if you are long loading too long, or even if you over crimped the bullet, ending up with the shoulders of the case slightly bulged.