[UR22 MKII] How to find "nominal level"

I’m looking to hook up my dbxb 286s to the UR22 mkII and the manual says:
“To get the lowest noise possible from the 286s, set the input attenuators (if any) on your recorder or mixer for nominal levels (often about 3/4” of the way up or at the detent position; refer to the device’s operating manual). Then, set the 286s MIC GAIN so that your recorder or mixer is at its optimal operating level”.

So how do I find the nominal level? Is this the so called unity gain-which I yet have to find, is it a fixed setting?
Someone on another forum suggested me to play pink noise and connecting the 2/R output to the channel 1 mic input with a TRS cable and to match them but I don’t understand how to specifically do that, any suggestions?

Thanks in advance!