The clipper does not care how loud your mix is
Worth clearing up first, because it is the opposite of what gain staging usually means. HardStop runs in floating point and has no internal headroom to run out of. Feeding it a hot mix does not overload anything.
What actually decides how much gets clipped is one thing: the distance between your peaks and the Ceiling. The signal path is short enough to hold in your head.
Input to Drive, into the clipper at the Ceiling, then Auto Gain, then out.
Drive is pre-gain, 0 to +24 dB. Ceiling is the clip threshold, 0 to -24 dBFS. Both knobs change the same distance from opposite ends: +3 dB of Drive and 3 dB down on the Ceiling remove the same amount of signal. Where your mix arrives only decides which knob you reach for.
So what goes wrong with a mix at -0.1 dBFS
Two real things, neither of them overload.
You lose your zero point. With peaks already against the Ceiling, Drive at 0 is already clipping. You have no setting that means "not yet", so you cannot tell whether you are 1 dB in or 6 dB in, and every judgment in how to judge a clipper depends on knowing that number.
The peaks are gone before you get there. A mix that measures -0.1 dBFS usually got there through a limiter, so its crest factor is already spent. The tall, brief peaks a clipper is good at removing have been turned into sustained loudness, which is the material a clipper is worst at: see what clips well and what does not. You are asking HardStop to shorten peaks that no longer exist.
So the useful target is not a number for the clipper's benefit. It is leave your mix bus unlimited and let it peak wherever it naturally peaks, somewhere around -6 to -3 dBFS on most material. Then the peaks are still peaks, and Drive at 0 means nothing is clipped.
Reading how much you are clipping
HardStop shows you the answer rather than making you infer it.
Find your zero first. Click or drag on the waveform to bring the dashed Ceiling line down onto the tallest peaks until it just touches them. That is 0 dB of clipping, and it is the only reliable starting point. The line and the CEILING knob are the same control, so the knob follows your drag.
Then read the gap. From there, every decibel of Drive you add is a decibel of clipping, and the red removed-energy area grows to match. The RANGE pill switches the waveform between -6, -18 and -36 dB views; -6 dB is the one to work in, because it puts the whole argument, peak tips against the ceiling, across the full height of the display.
Leave AUTO GAIN on while you judge. It applies the exact inverse of Drive after the clipper, so loudness stays put and you hear the character change instead of a volume change. That is what makes the comparison honest, and it is also why Drive is not a volume knob.
The procedure
- AUTO GAIN on. TP SAFETY off for now, so you are hearing the clipper alone.
- Drag the Ceiling onto the peak tips. Your zero.
- Add Drive a decibel at a time. On a full mix, 2 to 4 dB is the useful range. A drum bus will take more; see clip the drums, not just the master.
- Check the delta. LISTEN on delta should be peak tips and nothing you can recognize. If the performance is audible in it, come back down.
- TP SAFETY on last, and set your export ceiling per it measured under the ceiling but still distorted.
If you cannot leave headroom
Sometimes the mix is a bounce and you are stuck with -0.1 dBFS. You can still work: pull the Ceiling down instead of pushing Drive up, which gives you the same clipping with a visible zero point, and leave AUTO GAIN on so the output does not drop as you go.
What you cannot recover is the crest factor. If 3 dB of clipping on that bounce sounds worse than 3 dB on the unlimited mix, that is not HardStop being fussy. It is the missing peaks, and the fix is upstream.