Regarding skinfolds,
To my knowledge, there is only one skinfold formula that is validated down to 7.6% bodyfat in women, a four site skinfold by Withers, 1987. Most formulas for women are generally not validated below 10-15%. This means that they should not be used on anyone less than leanest female in the study. If you do use them, odds are the values will be less than they really are. We found that in the leanest women we studied (4-5% using DEXA and UW), if we used a 6 site women's formula validated to 10%, the leanest women were producing NEGATIVE % bodyfat values (i.e., they were way off). The best way to measure body fat is still UW weigh or DEXA on lean women if you want accurate numbers or two follow changes due to diet, supplements, etc.
W6