First I would ask you to check out this thread, and specifically #6 (post linked to in the thread): http://www.mp3car.com/lcd-display/15...ml#post1488680
Your guage resistor will be either Z1 or Z2. The other resistor you select will complete that diagram.
Voltage Dividers: http://en.wikipedia.org/wiki/Voltage_divider
software both available from me and 3rd party.