I had a chat with Pro BB Lee Haney about 8 month ago, he was visiting Dorian Yates temple gym in Birmingham - he told me that to measure b/f as best as poss by sight to use the following formula -
Top 4 abs only visable when flexed = 10-12%
Full abs are visable when flexed = 8-9%
All abs visable when relaxed = 5-7%