Inverted Analogue Rangefinder

I have a Sharp 20cm-120cm IR Range Finder Sensor and I am trying to get it to work.

0.5V at 1.2m and 2.5V at 20cm. So basically it’s higher the closer it is.

Not sure how to set it.

Do you have the RNGFND1_FUNCTION set to 1 (inverted) by any chance?
If not take a look at the value of your offset in case it’s non zero or negative.