Linea Research Limiters - Help!!

1. The VX threshold is in volts on the linea video but on my system engineer software its shown in dBu. I have no idea what this means, is this dB relative to some standard? Voltage is way easier for me to understand. Is there any way to convert this to voltage? 

2. Limiter attack on the RMS limiter. The linea video says it should be set to '100/frequency of high pass'. What unit of time is that? For a 40hz high pass that's 2.5 seconds or 2.5ms, this seems quite long or quite short. Powersoft guide says 'Typically a good compromise is setting an attack time no longer of the lowest frequency you have to protect (for example, 1 ms for 1 kHz).'  For 40 hz this is 25ms which is totally different from either of the Linea results. What should I be setting this too, I'm totally confused.

3. Would I be correct in assuming that the VX overshoot should be used to keep the amp out of clip? or should I be using some other limiter setting for that?

Is someone able to give me some example settings and their logic behind them for a punisher and  DE250 just to get opposite ends of the spectrum? 

Punisher 800 watt rms, high passed at 40 hz Powered by a QSC PL380
De250 80 watt RMS, Highpassed at 3.1khz  Powered by a Jam Q7 

Thanks

Hugh

It isn't as simple as plugging a limiter value in, there are separate peak and RMS limiters that need to be programmed independently of each other. Id also like to understand how they actually work rather than just plugging numbers in and hoping it works. The Linea documentation is quite vague.