Hi, I shoot 9mm major (Armscor brass, 7,0 gr 3N37 powder, 124 gr RN bullet, CCI SPP = 168 PF) in my BA UR and have an 8 pound recoil spring and shock buffer.
I've tested everything from a 7 to a 10. 8 felt best and shoots flattest for me personally. "Correct spring" depends a lot on shooter's preference, to include your grip.