Uh, isn't the whole purpose of the BDC to adjust "automatically" for distance, by simply changing to the correct number? I think if you go to Nikon's site, it gives instructions (as should have come with your scope) on how to do it. They have a calculator as well. Basically, you'll end up with something like 1 being for 25 yards, 2 for 50, 3 for 80 4 for 120, etc. that kind of thing (those numbers are all made up by me, but I think that's the idea of it.)

Here's a Manual for a BDC, although it may not be specific to yours.