Determining Your Maximum Range
Once you have your POI and POA lined up, give your barrel and choke a good cleaning. I normally run a BoreSnake down the barrel a time or 2 between shots and tend to have better patterns with a clean barrel and choke. It can be tough to get a BoreSnake to pull through a turkey choke. If you are having problems, switch and use a 20 gauge BoreSnake for your 12 gauge. Do not run a BoreSnake down the barrel without a choke. That will fill the threads with debris.
I should point out that there are some guns will actually shoot better dirty. You will have experiment with yours to find out. One clean shot and one dirty shot back to back will probably tell you the story.
Next, you should tape or range 40 yards and shoot multiple patterns. Actually measure it because paces or long steps could get you into trouble later. Sets of 3 patterns would be best, because it will show you the variation for your setup.
Count the number of hits in the densest 10-inch circle for each pattern. If the pattern counts are greater than 100, then your choke should be effective to that distance. Move back in 5-yard increments and repeat until your pattern drops below 100 hits.
Now you have figured out the maximum range for the conditions on the day of shooting. You are not done. The maximum effective range of a choke is not a constant, because your pattern numbers are not a constant. It is something you need to determine every time you go hunting. You will need to do a lot more patterning to understand your maximum range. It will change with the conditions. Cold temperature seems to affect patterns more than anything else. Rain and swirling winds will also reduce your pattern density in the field and will reduce your effective killing range. I would recommend that you do a lot of shooting and practice determining your maximum range under different weather conditions. Test yourself to see how you would have done in the field. This is something you can do all summer.
In the fall or spring, shoot at the coldest temperature you will normally hunt in and jot down the pattern numbers you got. In the summer, shoot in 75-85 degree weather and you will probably see the best patterns possible out of your choke. Every time you shoot, write down what you got and what the conditions were so you can learn how to adjust in the field.