The MCP4231 IC I have have two 100K pots. I agree-- If I put a 100K resistor between the pot's ground terminal and ground, the wiper output would be 2.5 volts when the wiper is a minimum. That is if the pot is powered with +5 volts DC. So to get the 1.8 volts you are subtracting .7 volts from the 2.5 volts. That would not shut the LED off but would make it a lot dimmer. Is that what you are getting at runaway_pancake?