I am building a bench supply. I have a 36 volt 9 amp supply that I was going to use as a voltage source. But it doesn't have negative voltage. I came across this circuit for creating negative voltage:

So that's good. I can use fixed voltage regulators for many voltages but it would be nice to have an adjustable voltage as well. So I am looking at this from a data sheet for the LM79xx:

Here is the link to the data sheet:

http://www.hep.upenn.edu/SNO/daq/parts/lm7915.pdfand this is the part I am having some difficulty with:

Look at the bottom of the page. There is a set of values for R2 at different voltages. Using these values I need to find R1 but the numbers I am getting are impossible. So I suspect I don't grok what Vset is and the only place you will find Vset in the data sheet is right there. I am using 36 volts for the supply, that is what I thought Vset would be, and I want a negative 15 volts out. How is this done? I keep coming up with a negative value for R1 and unless things have changed a lot I don't think you can get a negative resistance potentiometer.