Monitoring negative voltages

From time to time, I have seen requests for information about how a PIC - using only a positive voltage supply, can measure negative voltages.
In order to simplify things, I have created a spreadsheet that does most of the work for you.
Those with better math skills could probably simplify the equations, but nonetheless, you can find it here:


http://www.charleslinquist.com/PICNegativeVoltage.xlsx