When patterning a shotgun, you'll need to shoot with the choke tube and load combination you plan to use for hunting or game. Larger shot sizes like buckshot will spread more out in a pattern, while smaller birdshot sizes for turkey hunting tend to be tighter.
You'll also want to make sure that your gun is properly fitted and mounted. A good stable rest is essential, and sand bags and shooting sticks work well for this purpose. Lastly, be sure that the weather is decent. Shooting in the rain or snow will greatly influence your results.
Most standard pattern percentages are based on patterns at 40 yards, but there's no reason you can't shoot your gun at longer distances to see how it performs. For example, many upland shotguns excel at 25-yard shots, so it makes sense to shoot patterns at that range.
If you don't know the choke tube and shot size used to create a given pattern, you should start your testing at 20 to 25 yards. This is typically enough distance to show a clear picture of the pattern and how it might perform at turkey range.
You'll need to do a lot of testing to get a complete picture of your gun's performance. This is because there is a lot of variation in shotshells, shot sizes and choke tubes. A new set of tests is required whenever you change any of these interrelated components, such as by switching to a different choke tube or using steel shot instead of lead upland loads.