8.	Meter adjust	Press the IN switch to power on. Turn the THRESHOLD potentiometer counter clockwise until the output level drops to 0.5VAC. Adjust trimmer T4 in order to read 12 dB attenuation on the meter.

so with a 2vac 1k tone, the threshold knob ends up at about 2 o'clock at .5vac. 

I do not have enough adjustment on the trimmer to get to 12 DB.