I recall a +/-3 % error rate in skinfold measurement. I think that's pretty common. The more you do, the better you get. Having the same person conduct the measurements each time also increases the chance of taking a consistent measure.
Hydrostatic weighing is known to be more accurate. Then again, look at your progress over time. Try not to dwell on week to week, but maybe month to month.
Check you local universities also. There h & pe dept.'s may also have the means to help you map out your LBM accurately.