My pleasure. The dominant uncertainties are summarized in the last couple pages. If you would like to know the main errors in ytterbium let me know. I can ask Dr. Oates next week.
The data you gather to determine accuracy is tedious, thrilling, gob-smacking, humbling, and generally fun. Here's what we measured to determine the accuracy (uncertainty) of NIST-7. http://tf.boulder.nist.gov/general/pdf/1497.pdf