I'm guessing like a +2db over the loudest midrange instrument or leave close to level?
A common trend I've noticed in your questions as you work to improve your mixes has been requests for absolutes and magic numbers, such as the +2db one above.
Being a fellow geek, I can certainly relate to the logical / rational side of your brain wanting to have specifics. I know life would be a heckuva lot easier if you could just pull up a spreadsheet with formulas for each song and detailed rules on how many dbs in volume, what EQ bands, Q width and cut / boost db, and so on. However, as I think you're starting to learn, mixing involves understanding concepts more than memorizing settings. What's absolutely perfect for song A is an unmitigated disaster for song B.
There's a common bit of advice in the world of DAW based mixing that you'll hear everywhere: "Mix with your ears, not with your eyes." It's so very easy to do the latter when you're sitting in front of the computer with all the information a DAW gives you, and it trains you to approach things in an unproductive manner. To this day I still fight this. After tweaking a level on something that ends up being -5.12, my OCD nature immediately wants to change it to -5.00 so it's a nice, even number. And I still catch myself watching the numbers or graphic display as I make changes and realize that my attention is on the visual and I'm not at all paying attention to what I'm actually hearing. Argh!
So, this is an ongoing challenge for anyone who's intellectually based, and it's pretty clear that you fall into this category as well.
Time to actually "study" songs instead of just listening to them.
I think this is an excellent insight and definitely shows growth.
To do this you need to study with your ears, not your eyes. You need to feel when the vocal or lead sounds like it's in the right place, so you need to learn how to feel where it is in other people's songs. This is an acquired skill, so you have to work at it, just like it's hard at first to learn the guitar part of a song off the record when there are fourteen different instruments playing (and five of them are other guitars). You have to learn what to listen for, how to recognize it when you hear it and then how pick it out of the crowd. It's the same with mixing as it is learning a song.
Another exercise that could do you some good would be to go back over all the mixing threads you've started. If you're looking for it, you'll notice common trends where people talk about the same things every time - reference mixes, room acoustics, using your ears rather than expecting there to be specific formulas or a magic technical trick that the pros use. An example of the latter might be the side chain compression mentioned above. Yes, it's a common technique, but your instinct (like me) is to get caught up in the details of the technique itself when the real magic of it ends up being the same old thing - after you twist the knobs, how does it sound?
I know this has got to be a bit frustrating in terms of advice. You're working very hard on improving and you're making great progress, but you often ignore the advice you get because it's not the answer that you want. What you want to know is the specific 1, 2, 3 of what knobs to twist and by how much, and all you get back from us are vague suggestions to use your ears and listen to how professionally released stuff sounds. That's because what you need to know is, "mix with your ears, not your eyes," so as friends we keep pointing you in that direction. Friends can be a bit pesky in that regard.
So, the answer to your question is certainly on this page, which includes mix a solo like a lead vocal, use reference mixes to hear how both are done, and find the sweet spot for the given song. Ducking other tracks, as well as carving out EQ to find space, are techniques. As such, they're step 2. Step 1 is knowing what the end result is that you want to hear. Then you use step 2 to accomplish that. First pillage, then burn. Order is important.
There was a great quote that I've never forgotten from artist Ralph Bakshi about contemporary artist and friend Frank Frazetta. Bakshi was talking about painting but I realized it applied to music as well. Late in life Frazetta had a stroke and couldn't use his right arm, so he started painting with his left. People were amazed that the work still looked every bit as good. Bakshi just shook his head and said, "Everyone knows that you paint with your eyes, not your hands."
Or, to use the sculptor's advice on creating a statue of an elephant, "Start with a big block of stone, then chip away everything that doesn't look like an elephant."
You've got great technical capabilities with both guitars and computers. Now it's time to do what you just said yourself. Start studying songs and learn how to pick out what they're doing - different levels, different EQs, panning, reverb, etc. Internalize that, and learn what it is that you're wanting to hear. That's the specifics you want to get a handle on. Then when you sit down to mix, close your eyes and just turn the knobs until you hear what it is you're wanting to hear.