I don't think there's any rule or manual. All Rodgers organs are shipped with factory voicing. Voicing on site amounts to adjusting that voicing to adapt to the room acoustics and also adapt to how the organ is going to be used. This voicing requires someone with a good ear and experience who has a good idea what this organ might sound like and what needs to be adjusted to get there. My organ was voiced by Dr. Robert Tall and that took a number of hours.
One part of voicing is evening out the volume of some stop. For instance, a pedal stop may be too loud on certain notes and too soft on other notes. However, voicing is more tricky when it involves the overall volume of a stop or the volume of a stop at the low end compared to the high end. When you're adjusting an 4' Principal for instance, it needs to balance other stops that are going to be used at the same time. So you might get the 4' Principal to sound nice and even from bottom to top but when it's used with the 8' Principal, it doesn't sound right. You can imagine how complex this gets when all the stops on a division have to work together.
John Mitchell
TM 958