A mod choke will give you average results no matter what size shot you use. For ducks, #4 should be plenty, but with a pay load as small as a 20 ga, you really need to pattern your gun. You will be amazed how different weight loads of the same size pellets will give you different sized patterns. The speed, size pellets, choke, even barrel length all factor into what your pattern looks like.
You will need a paper target 30 inches across, and start at 15 yards, go to 20, then 30 yards. Aim for the center of the paper, use a rest, like a shooting stick and aim center mass. Measure the MAIN part of the pattern. You will have flyers, but dont count them.
This may seem like a lot to do just to hit a duck, but I promise you, it will teach you a lot about your gun. I have a CZ 720 I use for doves. The first time I took it out, I couldn't hit anything with it. After patterning it, I found out where my point of impact was at given distances, where my point of aim needed to be and that at 35 yards, my pattern was only about 20" across.