Congrats, you're doing something that 80% of the waterfowlers never do, shoot some paper. It can be an eye opener.
But you need some standards to follow and make sure you can make the correct decisions. Fortunately you're in luck cause I'm anal about this stuff!
If you follow these basic standards you will be able to decide what choke and shot size is best for the ducks you are targeting at the distance you'll be shooting.
When deciding what size shot to use here are the only reliable guidelines that are available to waterfowlers. Gel Penetration from KPY Ballistics.
"Here’s the info from KPY Ballistsics for folks to compare. I used 1450fps for all loads because that is decently achievable and is a published speed for all these. I calculated for 1.5″ ballistic gel penetration which is commonly accepted yardstick for mallard ducks, and for 2.25″ gel penetration, again a common yardstick for geese–whether you think that is a correct penetration or not is immaterial to using a single standard calculated penetration to compare loads. I’ll give pellets in 1oz so you can calculate easily other payloads."
shot size pellets/1oz 1.5″gel 2.25″gel
Steel #4 190 pellets, 31.0yds —-
Steel #3 152 pellets, 38.8yds —
Steel #2 124 pellets, 46.2yds, 24.0yds
Steel #1 102 pellets, 54.4yds, 30.8yds
Steel #B 85 pellets, 62.8yds, 37.7yds
Steel #BB 71 pellets, 71.5yds, 45.0yds
Steel #BBB 61pellets, 80.7yds, 52.7yds
So, as you can see if you're targeting ducks, #3 steel gives you the desired gel penetration out to about 39 yards. #2 steel gives 1.5 inches of gel penetration out to about 46 yards. If ducks are in your face, #4 steel will work.
When shooting paper it is important to have a standard to follow, there is good info available for the 40 yard shot and the number of pellets inside the 30 inch circle. This 30 inch circle has been a proven and accepted size of what it takes to reliably kill a duck or goose. This is why you shoot and count pellets inside the circle, it gives you a standard to follow so you can make informed decisions.
You'll need to shoot at paper at least 40 inches by 40 inches, bigger is better. To help eliminate any human error draw the 30 inch circle AFTER the shot around the densest part of the pattern.
Now count the pellets inside the circle. Since we know how many pellets are in any shell you'll be using we can calculate the % of pellets inside the circle compared to how many total pellets you're shooting. For an easy math example let's say you're shooting a 1 oz load of #2 steel shot which is 124 pellets and you got 93 pellets inside the circle. 93 divided by 124 gives you a 75% pattern which would be considered a good pattern.
However, if you only got 70 pellets inside the 30 inch circle with that load it would only be a 56% pattern and you would definitely know to try a tighter choke cause there are 54 pellets outside the 30 inch circle that are being wasted.
So, find the choke that gives you anywhere between a 65% to 80% pattern at 40 yards.
Finally, how many pellets does it take inside the 30 inch circle to reliably kill a duck? The highly regarded Tom Roster came up with this chart and it is by far the best info available.
This chart shows that it takes 85 to 90 pellets inside the 30 inch circle to make a clean kill of a large duck over decoys.
If most of your shots are at 30 yards this job is easy, if you're gonna have shots beyond that 40 yard mark keeping the pattern together at longer distances becomes more of a challenge.
Just remember, it takes 1.5 inches of gel penetration with 85 to 90 pellets inside the 30 inch circle, at whatever distance your shooting.
Since you are just starting out on this venture it is common to ask, Where do I begin with all this info?
I know a little bit about your SBE2 and would recommend starting with your Factory Modified Choke using #2 steel shot. Shoot at 30 yards and count pellets then move out to 40 yards. Then if you wanna shoot the after market tubes you'll have info to decide if it's worth the cost and time of chokes. My guess is you'll be pleasantly surprised at how well the factory chokes pattern. The SBE2 is a fine quality weapon.