it always depends on the incoming signal.
Looks like you can drop that -20dB attenuator
I use an st-9100 from Church Audio and I set the preamp so that highest peaks ever seen so far are above 5dB. At start I set the gain to +15dB and if the show turns out to be louder than expected, AGC Safety will deal with it...
Setting the preamp is a bit of guessing, you might have some bad shows at the start until you get a feeling of the right values to use.
Even the very good stealth recorders started off with bad recordings, you just learn and get better