Presumably, you have a mono switch in your system, probably at the preamp. If you use a sound level meter in the middle between your headphone speakers, you would play mono and adjust the balance to a maximum on the meter. I have the Radio Shack analog sound level meter, and I don't think it is precise enough to work with headphones within 0.1 db.
In my opinion, the best way is to put on the headphones, play a mono signal, and adjust the balance until the sound appears to come to you exactly in the middle of your forehead. This has the added advantage of centering the signal for any differential in your ears' sensitivity, as well as the headphone speakers. It may not be the ultimate in precision, but any imprecision is below your level of detectability anyhow. Good luck.
In my opinion, the best way is to put on the headphones, play a mono signal, and adjust the balance until the sound appears to come to you exactly in the middle of your forehead. This has the added advantage of centering the signal for any differential in your ears' sensitivity, as well as the headphone speakers. It may not be the ultimate in precision, but any imprecision is below your level of detectability anyhow. Good luck.