After making the changes discussed in the previous posts, I tested the precision by measuring the same oil (Extra Virgin Sample 1) 3 times.  Here is the result:

I also re-tested the different oils.  I'm surprised (and concerned?) by how little absorbance was detected in the canola and light samples.  But in general I think the results are decent.

Lastly, here is an updated illustration of the spectrum-finding GUI.  The previous version shows inadequately collimated light; the issue is visible in the middle of the spectrum.  This version doesn't seem to have that issue.