Q: How should I set the gain make-up control on my compressor?
The gain make-up control on a compressor is mysterious in function. Why so? Because it doesn't make any difference to the sound, that's why. So you can tweak it all you like, it won't make any difference to your signal, other than make it louder or quieter.
So why have it?
One of the keys to successful recording, editing, processing, mixing and mastering is to be sure at every stage that you are making progress, not anti-progress - or regress, if that's a noun (It is now!)
So whenever you do anything to the signal, from moving a microphone onwards, you need to check that you're making things better, not worse.
A classic case of this is in EQ, where you can easily play around for ages and end up with something that's a dog's dinner compared to the corned beef you started with. You thought you were making caviar, but it didn't turn out quite that way.
In the 'olden days' of audio, budget mixing consoles would lack EQ in/out buttons, to save cost. So it wasn't easy to check whether any EQ you applied made an improvement. But with an in/out button, you can hear straight away. If you want me to tell you how useful this is, give me Â£100. There - that's a demonstration how just how useful. And you got a good deal.
The same applies to compression. How can you be sure that you're making things better?
Fortunately these days all plug-ins have in/out buttons (anyone know of an exception?) so you can easily compare uncompressed versus compressed.
But you might not be comparing like with like. All other things being equal, the compressed signal will be quieter than the original. So how can you tell if it is better or not?
The answer is to turn the gain make-up control until the compressed signal is subjectively the same loudness as the original. Now you can flip back and forth and easily tell whether you are working along the right lines.
The best thing is that you don't have to worry about the gain make-up setting! It's only there for your convenience and doesn't make any difference to the signal other than level, and you will set that with the fader in any case.
By the way, you might consider looking at the gain reduction meter. Look for the maximum number of dB it displays as it bounces up and down. That will be roughly the right setting for the gain make-up control. But don't take that as gospel - use the setting that works best for you.