+0.5 should be best guess at the moment. I was able to get mine only to 0.0 My car had -2.2 to -3 in some situations and I need to jump one tooth over to get more adjustment. That had huge effect to low end torgue.
I don't have the Audi anymore. I changed it this week to a 2012 Seat Ibiza ST E-Ecomotive 1.2 TDI.
The Audi had travelled over 350 000 km so the list of repairs for the was quite long, and last week the automated manual gearbox started acting strange so I felt like I should change to a car with more room. The Audi didn't have much luggage room because the rear seat didn't fold down and it didn't have a trailer hitch.
It sounds like you need some basic tune up maintenance. My first thoughts upon seeing the car included some tail work, belly pan, and wheel covers. Some thinner oil might help if it is ok to use it. Have you looked into water injection?
I'm no diesel expert, but some ecu tuning might help too.