Within the mobile app, you can record negative readings for brix values as part of the analyses on each lot.
To enter a new value, select the "Analysis" button under the "Record Action" dropdown on one lot. Chose Brix as your analysis along with any others you are running. Enter the positive form of the Brix value in to the "Value" box. Press the "Flip sign" button to change the entered value to its negative inverse, then press Save to retain this value.
Once you see the correct value entered next to the Brix header on the Analysis page, you are all set to press the "Record Analysis" button to get your Brix value into the system for that lot.