The "FBI" in "FBI cant" stands for "forward body index", as in the holster cants toward the front of your body. 15° is pretty common, but there's no set standard and it has nothing to do with the Federal Bureau of Investigation.
Mine was a unique problem.
I was blessed with an extremely long barrel. So when I tried to conceal it, it was a hard task to master.
At 3:00 to 3:30 my tip would stick out by my butt cheek like I had a tail. So low that when I sat down, it would push up and try to unholster itself. Used a hybrid but it needed to raise the gun up more than the adjustments allowed. I had the holster shell raised and refitted to get some more adjustment.
That did it. With the gun now raised to a comfortable level, I could canter to an acceptable way so the grip is maxed forward not to print and the barrel maxed back enough to hide and for me to function normally.
It's so comfortable now that I sometimes don't remember I'm wearing it.
Conceal Carry my angle is about 25-30 degrees.
Now when I Open Carry, I just let everyone see my truly magnificent gift.
I still have a slight angle at about 15 degrees when OCing.
This is actually the first time I've ever measured it..............the angle that is.
Is there an OWB horizontal holster for full size 1911 that you would recommend?