Here is a recording with an image captured from a Wav Editor. The red line represents the level where the threshold setting should be when I apply compression on the signal in thatfile. I never understood this: how do you calculate where you need to set the Threshold? You can see there is a 0 to 1 view on the meter in the editor. But in compressors, the 0 is on top, so that's a complete reverse, and you express the threshold like - xx db. But how to figure that out from the editor view?

 

Typically that doesn't make sense, because threshold is just one of many parameters that affects the peaks. You should also be able to just switch the scale from these linear values to dBFS or whatever you want. That being said, it's usually dB = 20 × log₁₀(V).
When your line is at 0.4 (?), this would be 20 × log₁₀(0.4) dB ≈ -8 dB.

 

I don't get the question, but I would love to.
I would like to understand why I would have to calculate something. The threshold is the value from that on the compressor does something.
It goes from dbfs minus infinity to 0. The waveform shows what the level is at that exact sample. The sample values go from minus infinity to 0dbfs.
If I set my compressor threshold to -18dbfs and there comes a signal higher than that the compressor is gonna do something.