I use the 2.3mm prong and have 11 links in it. I put in on at the top of the neck behind the ears, but after awhile it slides down to this position - but doesn't go any lower than this.
If it starts getting to this point and I feel him pulling a bit more, I just schooch the collar back up behind his ears.
I agree that you don't want the collar too tight, or that would defeat the purpose of self-correction. I know if I took a link out, it would be too tight. I had this prong on Red all day and it worked really well to keep him from pulling, yet he didn't act like it was constantly uncomfortable to wear either.