Opossum,
You never did mention which game you are shooting. I was watching the guy who won Olympic trap on Shooting USA the other night, and he was talking about chokes. He said that Full was good out to 40 yards, IM to 35 yards, Modified to 30 yards etc, in five yard steps for each change in choke.
I'm with Greybeard, I use modified from the 16 yard line. In fact, I use modified from the 21 yard line....but it kind of depends on how quick you hit the targets. I tend to hit them pretty quick, but not as quick as some guys I know. I also know guys who don't get them until they are almost on the ground. They need a full choke.....maybe even a turkey choke.
It also depends on the gun. I'm shooting a Krieghoff now, and it tends to require less choke than my Browning did.