I use winchester white box 115 grain fodder. I bought a woff calibration pack and tried them all. Settled on the 12#. All the weights worked in the gun, though weights over 14# really slammed the slide when it came back into battery and weights under 10# seemed very sluggish on the return. I felt the 12# was the best compromise.
I agree with Ted the 12 seems to be correct in most guns, although a few of my 9's run an 8 or 10 lb. spring. As a general rule I run a 12 with a Wilson buffer but haven't replaced the buffer in 3+ years and have used the gun for almost all IDPA competitions.
Great timing on this question. I had planned to purchase a couple springs to play with in my Springfield. I was going to look and see what factory is and then go up and down from there. I am sure mine is not close to factory anymore. I have never replaces it.
Let me know if you try anything and I will do the same