Having read many sophisticated posts from you before, Ill be surprised if you dont know this, but Ill add it just in case anyone stumbles upon this thread…
Right, gain stage properly.
But before that, one thing a lot of people dont know, in part because there is real inconsistency by manufacturers , is whether the line level outputs have ts or trs cables. Trs can save you some noise on some units if available.
Your Focusrite of course will take either.
Another noise contributor is dirty power. I have mostly had this live but it is an issue in spots that were not renovated in the 100 year old house I live in. They are all grounded, I checked, but there is just micro fluctuations. A Furman power supply meant for this helps.
Once you have noise low enough, you can just not spend too much time worrying about levels, digital has so much headroom. I aim for anywhere from -20 to -6 depending on how transienty the content is. Bit I dont spend a lot if time on the front end worrying about it like I did with analog or still do with my hardware peripherals