How to reduce volume by 30%?

[thoste]

I want to re-encode a loaded *.mp4 video (with H.264 video codec).
In particular I want to reduce the volume by 30%

How can I achieve this?

I selected AAC (lav) from Audio Output drop down and then in Filters

Gain: Manual (db)
Gain value 0,7

But it didn't help.

Is gain value= 1.0 = 100% = current volume level?
Then 0,7 must be 70% target level.
Or do I have to use negative values?

[sark]

Gain adds, or deducts dB to the volume. You can only control percentage by determining the current volume and calculating the required change. AFAIK there is no way of accurately determining the current value with Avidemux.

If you have Audacity installed, you could use Analyse RMS to get the value.

[eumagga0x2a]

In particular I want to reduce the volume by 30%

How can I achieve this?

Apply manual gain of -3 dB.

[sark]

In particular I want to reduce the volume by 30%

How can I achieve this?

Apply manual gain of -3 dB.

Whilst audio is out of my area, I'm pretty sure dB is logarithmic, not linear.

EDIT.
            OK, I did some maths and it is indeed -3 dB (-3.098039 to be exact). It is important though, that the OP does not assume Gain is a linear computation.