In the BFL Forum (under the Exercise forums) there is a sticky called "Using BF% as goal". You'll find a chart attached to the first and second posts in that thread with information on desired BF% at differing weights. 22% BF is considered 'good' anything lower is, well, better - up to a point. A BF below 17-18% and most women start to experience irregularities in their period and other health problems.

HTH
Nat