there are 2 trimmers on the pcb, for zeroing out the bleed on each vca. you can get all scientific and measure the db. but I just do it by ear. insert a harmonically rich waveform like a saw or square into the audio input with the panel control/knob at full ccw(counter clockwise)and plug the out to whatever mixer/amp chain you work with(maybe turn your mixer/speakers up louder than usual) and then turn the trim on the pcb to the left/ccw until the vca is silent. If your vca is silent from the start then good, and move on and repeat for each channel.
Motherboard is same as dual vca. See dual vca build docs.