Ruby, I am doing a similar thing with oranges, and as you say, I don’t have Brix numbers until after I have scanned the cut oranges and then juiced them.  I am creating sample identifying attributes which are mandatory, and then other attributes (such as Brix) that are not mandatory, so I don’t have to put in dummy figures before hand, I just go back in and enter the figures later.  Saves some of the messing around that you are dealing with.