Loudness is always calculated based on damage (with a possible exception for gunmods, which I believe have their own “loudness” values). The reason the results might seem strange is because there is a spike right at 7 damage, where the noise amount shoots up by around 20 or so. At the moment there is no way around this.
Currently there are three different UPS flags. USE_UPS (5 regular, 3 advanced), USE_UPS_20 (20 regular, 12 advanced), and USE_UPS_40 (40 regular, 24 advanced). In order to add other values you would need to change some of the source code, and is beyond the scope of json modding.