Hi,
First of all the level you can see is a VU which means this is not exactly related to the Peak level (to see pseudo peak level change to PPM).
Various music and pplications require different average levels. The best way to find out what VU level are the best is to listen through the VintageWarmer some commercial music of the same kind you make with a Drive set to 0dB and Knee set to 0%. Then go to the back panel and adjust 0VY reference level so the reference music would drive meters to 0 and occasionally to +3. After that calibration close the VintageWarmer and every new instance of that plug-in will be calibrated in the same way.
Now you can go back to your music and see what VU levels are displayed.
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Mateusz Wozniak
PSPaudioware.com