The max sound level you're planning on going SHOULD be set at 70% you should never PLAN on going above 70%. the headroom is there to allow you to increase volume when the track is just too plain quiet.headroom not to clip things when you are pushing it to the maximum sound level you're planning on going.
If you set your maximum sound output at 100% you've just sound checked and run unity at 100% instead of 70%
If you need it quieter than the max output (which should be 70% ish) then then just turn it down below 70%
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