Regardless of the PA, you need to set each component, source, mixer, amp, speakers, to the correct and optimum gain stage. In other words, no component should be over driven, so the sound is clean at each stage.
Check each channel of the mixer by inputting the loudest mic or sound to be sure it is overdriven.
Check the output of the mixer and the inputs of the PA to be sure nothing is over driven.
Ideally you should have headroom at each stage. If you don't have enough volume, you need a larger amp or active speaker. There is no magic percentage to go by as almost every setup is different.
On my own setup my mixer never goes into "red" on any channel. The lines from it go into two separate powered speakers, and they have plenty of headroom for anything I want to do.
I set the speakers so that they won't distort or go into safe mode. If in a venue where I need to be really loud, I can pump up the volume from the mixer because the speakers are set where they can't be overdriven.
Hope this helps.
_________________________
DonM