LM3914 LED driver

Hi -

I am incorporating a LM3914 IC into one of my projects. The current schematic for the 10 LED battery indicator is attached. This however assumes you want to measure voltage range of 0V - 5V. To adjust this range new resistor values need to be determined.

I need a range of 3.1V to 4.2V as the project will be powered by a lipo battery. I have not been able to successfully calculate the values for R1 and R2 in the schematic and would appreciate if someone can assist?