Checking (or setting) POI is best done at 13 yards. By using this distance, you get a more accurate idea of just where the the POI is in relation to the point of aim, since the "pattern" at that distance is more of a single point of impact or hole, and the distance up or down (or left/right) from point of aim is more easily determined. This is done usually by shooting from a rest, to eliminate errors in holding, etc so that it is the guns POI that is determined. To then figure the approximate 40 yard POI, just multiply the distance determined at 13 yards by 3; i.e, if the POI is 1 inch high at 13 yards it will be about 3 inches at 39 yards. This will get you to a set POI using your comb adjustments to set the POI. Then, if possible, I find it best to go to the range and set the trap to simply throw straightaways. Using post 3, I then check my POI by shooting the straightaways and observing hits. If the largest pieces are going upwards, then I am hitting a bit low and can adjust POI up a bit. If pieces are going low, I am hitting a bit high, and will lower POI a bit. Continue this process until you are centering the targets.
Jim R