I tend to optically centre about 50yds, because I don't tend to return to zero after a shot, and so 45 is about average target length, so i'm likely on average to have left it there with longer shots being usually the 2nd target over here. But to be honest, i'm not sure it makes that much difference, just if you can, why not? Also I tend to do it just so i know i'm not going around the turret too far and running out of room... with 55 i know i've probably got another turn, turn + 1/2 , to get down to 8yds... and i've probably got just 1/2 a turn to get to 25-30... that means I can guess indoors, and centre it roughly to that before going outside and then finding i'm out of elevation.
On my march i've got bags of room, but something like my deben has a narrow elevation range which makes it more critical.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep