You can use the Nyquist prompt. and multiply the sound with a constant.
1 is equal to the momentary value or 100%. So, if you want to reduce by 10%, you can set the constant to 90 / 100 = 0.9.

You could use Effect > Adjustable Fade and set Start and End to 90 with the “Start/End as” as “% of original” - this does the job without typing a command.

I will leave the experts to decide if there would be any difference between using Fade Up, Down or their S-Curve equivalents. I would not expect any audible difference.

Do you want to vote for Amplify to have a % control for “Amplification (dB)”?

I wonder what your application is. Audio and your ears work in dB, not percent. 50% is not half volume. 50% is 6dB out of the 60dB that you can hear. Koz

A 10% decrease in amplitude seems like it should be quite a lot, but it is actually less than a 1dB change which is a barely audible difference in “loudness”.
Try it in the Nyquist Prompt effect - Robert J. H. has provided the code.

Percentage can actually be a little confusing… If I say 10%, I have to make it clear if I mean a 10% increase or 10% of the original. People sometimes get confused whan change in speed/tempo… i.e. If I enter 100%, does that mean no change, or does it mean twice as fast?

Also, the percentage change in voltage or in the digital file is different from the change in power (Watts)… A +6dB change doubles the sample values in your digital file and it doubles the voltage (200%), but it’s 4 times the power (400% ).

I need a way to enter percentage values.
How does this work?

In case you don’t know how to calculate dB - dB = 20 x log(ratio)
or ratio = 10^(dB/20)

With power (Watts) - dB = 10 x log(ratio)
or ratio = 10^(dB/10)

In Excel (using percentage) the formulas look like this: 20LOG(A1 * 100)
10010^(A1/20)

Where cell ‘A1’ contains either dB or the percentage. It this case, it’s the percentage OF the original, not the percentage change… In other words, 100% is no change (NOT a 100% increase) and 200% is twice as much.

I just made quick chart in Excel and it looks like this:

1 % = -40.0 dB
10 % = -20.0 dB
20 % = -14.0 dB
30 % = -10.5 dB
40 % = -8.0 dB
50 % = -6.0 dB
60 % = -4.4 dB
70 % = -3.1 dB
80 % = -1.9 dB
90 % = -0.9 dB
100 % = 0.0 dB
110 % = 0.8 dB
120 % = 1.6 dB
130 % = 2.3 dB
140 % = 2.9 dB
150 % = 3.5 dB
160 % = 4.1 dB
170 % = 4.6 dB
180 % = 5.1 dB
190 % = 5.6 dB
200 % = 6.0 dB