My goof. I had the ratio on the temp/resistance backwards in my head.
When the IAT reads higher temps, the resistance is LOWER. In that case, you want to INCREASE the total resistance in the circuit.
-40°F (and Celisus coincidentally) is 100k ohms. That'* the coldest value listed. For comparison, 212°F is 177 ohms, and 59°F is 4450ohms.
Add a pot in series with the IAT. Not in parallel. If it'* 95°F outside, you are running at 1.8Kohms. By adding in about 2.6k, you can make it think it'* 59-60°.
Adding in a 5K pot, cranked all the way would be 41° indicated.
There are some unknowns here. How much colder do you need the IAT to read? A 2.5k or a 5k pot would certainly do the trick. On another note, you don't really need a pot, but rather just a rheostat, although a pot can be wired as a rheostat, and by picking which tap you want, you can have it dial either clockwise or counter-clockwise to achieve your results.