Barb writes,

<< You will probably get different percentages from different samples,
 depending on time of day it was expressed, whether it is from the
 beginning or near the end of a feeding, what the interval was from the
 last feeding, etc.
  >>
If you are going to do this, you need to get a 24 hour sample; some from each
feed, some taken from the beginning of the feed, some from the middle, some
from the end -- equal amounts from each (say a capillary tube of each sample).
Then mix the entire 24 hour sample and spin down to get an appropriate
reading.

Jan