Sounds like you are right on track for getting it close. What I did, back when I used a scope for a while, was do just as you did at close range, then when I wanted to get the pattern centered at 40, I would draw a 10" circle then draw a 3" circle in the center. Draw cross hairs right down the center of the 10" and 3". Color the 3" so that it gives you a good visible aiming spot. Get your gun anchored good on a bench, and center your scope on the crosshairs on the target and squeeze off a round. Count all the pellets in each quadrant, that will show you how the pattern is centered and which way you need to adjust. I tried to get the quadrants as equal as possible. Others may do different methods, but this one worked for me.
Ridge Rooster