dap is like many Grouse hunters and patterns at 25 yards, I pattern all my shotguns with different shells at 25, 35 and 40 yards, using a Grouse sillouette. Some shells pattern very very well out of my L.C. Smith double guns, some do not. In fact my Churchill 20 guage leaves a big gap
high right with 7/8 oz AA's at more than 35 yards, for some unknown reason. All my 16 Guage L.C. Smith double guns
pattern extremely well with 8 or 7 1/2 SpredR's in the C BBl and Fiocchi 7 1/2 or 6's in the IC BBl. The patterns are complete and deadly on flying birds from 25 - 45 yards. Definitely take the time to Pattern your shotgun with several different manufacturers shells, if you want to fully understand your guns capabilities.
Pattern @ 40 yards....410 is normally less, as standard.
The distance tho depends upon what you are seeking to gain from the pattern.
If you want to check if a tube or barrel is delivering the marked choke percentage then use 40 yards.
If you wish to check stock fit then 16 yards is appropriate.
If you wish to find a good woodcock load, for example, and are looking at pattern spread rather than density then 20 or 22 might be the bee's knees.
As stated, some use the distance they believe they shoot most birds...that tho is too imprecise, to me....waste of time. Much like shooting at an old oil can.
It works tho, after a fashion.
One can get in the weeds as much as you want patterning....myself, I look at placement re point of aim, spread and the outer 5" ring of the standard pattern re most gamebirds or targets.
Inner core density rarely fails today.
Decide what you want from the pattern and stand at the appropriate distance.
Have fun, don't get too wrapped up in the process tho it can be fun and considier that any pattern is almost never the reason we miss a scattergun shot at a flying gamebird.
We miss because we shoot too quickly or lift wood from wood.