I don't think it matters whether you sight in at 50 or 100. I think they start at 100, but that's just a starting point. The BDC reticle is for "practical" situations, not precise target shooting. As long as you're within 3" or so in a hunting situation is all that you'll need out of the BDC reticles. My Nikon rifle scope with BDC gets me close enough (when sighted in at 100 yards) to pop a Coke can at 200 and 300 yards, respectively, with my 375 Winchester. Can't tell you exactly where it hits everytime, but it hits it 2 out of 3 shots.
Just sight it in and do a little experimental shooting, you'll see what works best. Try sighting it in at 50, then shoot it at 100. See where it hits and decide whether you need to hold over with the first circle. 5 - 10 shots should get you clued in to what works best. The BDC reticle is just so that you'll have a referrence point when you're out in the field.
Poll