King,
As a general rule of thumb, .6 is a good starting point for 100' of wire out. From there, you'll want to add .01 for each foot thereafter, i.e, 180' = .680. The voltage will decrease further down the wire. So with the box set at .680, you'll be in the .600 ball park.
Another factor to keep in mind... how far back from the wire you run the hooks. I run mine approximately16 feet (48' release + 48" for flasher and lure + 8 release to flasher). To do this, take a black marker and measure 8' forward of the flasher. This will keep the fish in the comfort zone.
Lastly, everyone has there on confidence zone with the Black Box. However, as another general rule, Blackmouth will like a higher voltage and Summer Kings like a lower voltage. Summer Kings I run right around .580 with a drop of 22' or so if fishing the upper 100' of water. Once the sun hits, I start applying the .01 formula to the box.
Heck, I though .580 was high until I learned John Keiser runs .650 in 60' for Summer Kings.
However, I am sure his drop back is a little further than mine.
Downriggin'
_________________________
"If you are not scratchin bottom, you ain't fishing deep enough!" -DR
Puget Sound Anglers, Gig Harbor Chapter