How to set output level for equipment measurements with REW and Scarlett Solo?

I am using the output RCA jacks on my Scarlett to send tones to equipment under test. I can adjust the output level in REW in increments of dBFS, to a maximum of -3 dBFS. I can also adjust the output level by turning the "Monitor" knob on the Scarlett. My instinct is to set the software to maximum and use the knob for final adjustment, but I'm unsure?
I am also attempting to figure out how to use the Scarlett Solo with REW and a Windows laptop.
I cannot figure out the wiring. Any advice or photos to help?

I am attempting to send the test signals from REW on my computer via the Focusrite Scarlett Solo to my receiver and have a MiniDSP UMIK-1 mic record the results.
If you are doing room measurements with REW and a mic, don't overcomplicate it.

1. Audio from computer goes out RCA from back of Scarlett into receiver.
2. UMIK-1 is connected to Scarlett and records room audio
3. Run measurement sweep in REW, playing over speakers via receiver, and recorded with UMIK-1

Don't worry about distortion, I think there is too much room for error to make that number meaningful. Normalize the results to at least 24 or less, you don't want all those jaggy lines. Don't obsess over small details in your measurements, unless you're trying to cure a particular peak or null. As soon as you actually go to listen, you're head and ears will end up in a place different from where you took the measurements, and all the minor variations will change. Focus on keeping the mic in a consistent position between measurements.
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