What magnification scope you using? The more mag, the more "boil" you can see. With some practice, you can calculate the change in POA/POI. No one is perfect at it, but it's too much of a variable to completely ignore.
When I shoot benchrest in AZ, or a state where it can get pretty hot, a 110 degree day and a 10 mph crosswind combined can put a 68 gr 6mm bullet as much as a 1" out of the group. It sucks when it's your last shot for record and the first 4 are in a .077" bug hole.
Try using a benchrest target, with a sighter target, a record target and a square aiming point. It's easier to divide a square with a cross-hair reticle than a circle. You know the dimensions, and if you need to hold a 1/2" or what ever for the wind and mirage, it's more repeatable.