The lowest dipped beam setting should be high enough that you can travel at about 40mph on a completely unlit road, User269 pointed this out earlier. With centre aimed headlights you should be seeing further up the road.
I'm VERY particular about headlight adjustment, it MUST be 1/2 to 2/3 of the way to the upper limit when dipped pattern is tested. This means even on on Fiat, which has the worst headlights of my cars, I have good enough illumination for 50-60mph depending how dark it is. The darker is it the better the apparent range of headlights.