Job one is record level in out of the a/d; On your peak reading meters in your app- average levels in and around -18 -- -24dBFS or so. Peaks should have a good safety margin well below 0dBFS (some say peaks at -10 is a good target. Straying above that some is likely fine, but that is what it's there for- space for your head room.
Now if and when our external devises (preamp in this case) happen not to align (operating level wise) with your a/d, oh for example pushing the pre for effect' makes your record levels off target, then you attenuate the line level going into the a/d.
Well you got to figure out your interface. For example, line level in with my saffire pro 40, I have to turn the knob up to 4.5 to get unity so 4.5 is actually 0DB.
The 0 on your analog meter is around -18 on your daw. Anything on the meter is fine as long as it's not in the red. If it's in the red, even if your daw isn't in the red, it'll be real distorted. I messed up some vocals one time because a guy was singing so quiet to a loud song. I could not get it to where we could even monitor him in the headphones without routing a special mix.
I reached over and starting cranking the pre amp and adding gain and it wasn't good. A professional would notice it. Of course I didn't go over much, but this unit doesn't have any headroom over 0db. It won't sound as clear.
Sometimes if you run into a situation where singing is low or it's quiet guitar playing in a mix with loud drums and all of that, you just have to start turning stuff down so you can monitor what is being recorded.