Peak TX Audio Level - Metering/Documentation

I find that I can only properly drive the radio if I have the peak level indicator in the red at 0 db, with the average TX audio level indicator peaking at around 75%.  Using a peak audio level of just below 0 db on peaks, as per the documentation (below), is a little too low for full audio and to drive the radio correctly.  

I use an external voice processor, but when using the radios audio processing feature you need to have the same setting as above to get some compression to kick in.

How much headroom before clipping at 0 db peak audio level ? 

Documentation;
When setting up your microphone audio for optimal modulation, adjust the input gain so that the peak
level indicator is peaking just BELOW the 0 dB on voice peaks. It is very important that your peak level
indicator never exceed 0 dB and turn red at any time.