Assuming your external compressor even has a threshold and all that, the correct setting is the one that gives you the desired result.
The real trick is knowing what to desire.
There aren't any presets generic to your voice, mic, dynamic control, type of compressor or gain staging habits.
Threshold, input gain, output gain, ratio, attack, release, knee, gain reduction...
They all go together.
The best thing might be to play with the controls and listen to what happens. It'll help to check with the compressor in and out of the circuit to compare. At equal percieved volume levels.
Some people like to track with those things for various reasons, but it's by no means necessary and possibly not really desirable.
Once you start mixing and using compression globally on a specific bus, your goals and approach to compression in general might change a bit.
Compression and reverb are a bit like glue in that sense. And you can never have TOO much glue, right?
The only way to learn is to do it.