252 out of 300 is fantastic shooting!
There is no substitute to build stamina. Repitition is all you can do.
I too shoot on a three hundred league, and I use the same set-up I use for hunting. I change nothing, except arrowheads.
This thread started out with you asking for help, but I would like to know what your secret is. Physically, I have no problem, but by the last few rounds my concentration is worse than a two year old after drinking a 2-liter Mountain Dew.
How do you maintain your concentration for so long?
After forty or fifty shots, I so much want to shoot at something besides a round white circle.