@pianopaul is absolutely right. Not only the loudness, but also the sound itself changes completely within a context of a band.
Sound can be moved left, right, forward, back and anywhere in between using a bit of panning and frequency filtering. FOH can determine what’s going on.
Having said all this - I do have a certain level I want to achieve for certain sounds in relation to each other. That’s important. So you can use your ears or a tool - whatever works for you.