What your forgetting is that you always load FOR YOUR GUN. The instructions are just a general guide. You always load for what your particular firearm likes.
Having said that in a semi auto in 9mm bullet setback can be a very dangerous problem since the 9mm will build pressure very quickly.
When starting off loading with a new firearm I always do the following.
Load up a dummy round (no powder no primer) start with a light crimp. Measure and log your OAL. Load into your mag and rack your firearm to strip the round from the mag and load into the chamber. Eject and re-measure. Compare to our original OAL adjust your crimp until your OAL doesn't move more than your comfortable with.
That's how you gauge how much crimp you need.
Here's an example from my own.
Norinco NP28 9mm 3/4 turn to keep OAL stable.
S&W 4506 45acp (sadly I sold it) barely 1/8 turn.
Springfield Arms XDM 5.25 40 s/w 1/2 turn.
These are my results. You need to find what works with your bullet and firearm.