From what I understand they are relative to “Standard Temperature and Pressure”
www.engineeringtoolbox.com/stp-standard-ntp-normal-air-d_772.html
To paraphrase Longacre; when you were really cooking, when everything was working just right and the engine was putting out what you thought was the best ever, when the RPM, at the end of the straightaway, was at its highest , write down your jetting and the air density. Use that as your baseline. After that, when/if the air density changes +/- 1%, adjust your fuel mixture +/-1%.
That’s why the gauge is read as a Percentage not a Value like Kg/M*3. It is the percentage of Standard Temp and Pressure at Sea Level (Standard = 100%).
Exactly! It’s meant to simplify the whole thing. Gauge changes X% then Jet flow must change X%.