Reduce 2.503V regulated voltage by just about 3mV
You can do this. The pot will be easy to set (range is only about 10mV). Output impedance is &lt; 275 ohms. Adjust values if that's too high.



simulate this circuit &ndash; Schematic created using CircuitLab
Of course if you happen to have a reference that is &lt; 2.5V or an offset that is in the wrong direction this won't help.