Bird Hunting is my thing so I can help ya
First things first get big peice of cardboard put it up 25 yards away. Aim very carefully and shoot the center of cardboard, put a black dot on it for a good aiming point. See how your gun patterns with improved cylinder, which is my fave dove choke.
Ok, that pattern with a 12 guage is about 8-12 feet long with a 20guage 6-8 feet long. So what your trying to do is put that little dove in the middle of that 30inch by 8 ft long pattern that travels at 1100 feet per second. The shotgun reloading books have great illustrations of this.
Now your in the Dove field mark a cricle around your spot 25 yards out with sticks or rocks, this is your kill circle. It will help you figure the distance of the shot. When you see a Dove coming in first figure his speed fast, medium or slow, for fast dove at 25 yards figure 10-15 feet of lead, for medium speed 5-10 feet, and for slow 2-5 feet of lead. Now for every bit angle he is coming towards you take off a hair of lead, if it is a pure passing shot dong take off any lead. You have to lead in both the vertical and not just the horizontal, this vertical lead is normaly what people forget to factor in.
Some other Tips,
Downwind dove are very hard they really fly fast try to position your self on upwind doves. One time I was Pheasant Hunting and bird took flight about 50 yards away and flew downwind right for me, the wind was gusting to 40mph so I figure the bird was doing 75- 85 mph, I knew I have to lead 20+ feet, but it just seemed so far, but I went ahead and trusted the math and pulled the trigger, boom I hit right in the head neck area, my greatest shot ever. This bird looked like it was moving about mach 1 and just folded it up.
Always swing through the target, never stop swinging.
Find a place with lots of Doves, pay if you have too, to get in a good field and spot.
If you find your shooting longer shots use 7 1/2's. Other than that skeet can help some for confidence.
Good Luck,