Post History

The poteniometers (what you label as "variable resistors") are almost certainly just floating pots. If their ends were permanently connected to power and ground, then they'd be voltage sources, not pots.

You might temporarily connect the top terminal to power, the bottom to ground, and measure the center with a voltmeter to set the potentiometer to the desired attenuation.  However, when you're done with that, you need to connect it in the circuit so that it attenuates the right signal, not produce some fraction of the power supply voltage.

In your case, you want to multiply a signal by 0.4.  First, you adjust the pot for the 0.4 ratio.  One way to do this is to connect its top pin to power, the bottom to ground, and measure the voltage coming out of the center.  If you have a 10 V power supply, for example, then you adjust the pot until its output is 4 V.

Without touching the knob, you now connect the pot into the real circuit.  Since you want to attenuate the output of an amplifier, connect the top pin to the amplifier output, the bottom pin to ground, and the center pin becomes the attenuated amplifier output signal.