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Here are a few basic points that should straighten things out:

Smoke is unburned fuel.

All diesels normally run very lean, but unlike gas engines, if they get close to 14:1 ratio, they are pouring smoke by that point.

Your TDI can run fine smokey and with no smoke, since it's already running very lean, lean is normal. Too lean and you can start to get a misfire.

The best level is such that your EGT is under control and to your preference. Most people don't want smoke under normal driving but some smoke under WOT is normal.
 

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I think what they mean when they say that is that it makes no noticeable difference in fuel economy. Of course if you putting out unburned fuel it's wasted fuel but we're talking minor differences. It comes down to personal preferences because tweaking IQ does makes a more noticeable difference in throttle response and smoke levels.
 
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