The temperature settings I provided you are the manufacturing set points. The adjusting procedure is to check the cam at two different temperatures. The roller on top of the fast idle cam has to be around the LEFT mark when coolant is 68-86°F and above the RIGHT mark when coolant is 167-185°F. If I have to chose between which mark to be more acurate, I would chose to align with the RIGHT mark at 167°F. If the marks do not position at those temperatures, then the thermo-element is bad and needs to be replaced. Last time I checked with Nissan they told me it cost around US$100.
In easy to understand terms, the function of the thermo-element is to regulate the engine RPMs from cold to hot. RPMs are set to high when engine is cold and are gradually reduced as the engine warms up. In your case, I believe, the thermo-element is sending the lever to the lowest idle RPM (approx 750) while the engine is still too cold, causing erratic idle. I am not sure if you remember this, releasing the choke valve too soon in old cars?
For a good adjustment, your mechanic needs a scanner to read the proper coolant temperature. If your mechanic does not have a scanner for reading the correct coolant temperature, then do not let him touch the setting and look for another mechanic that has a scanner.
By the way, I re-adjusted the fast idle cam in my van about two years ago. I did it because I disassembled the whole throttle body to take care of a coolant leak I had in the thermo-element. I replaced the O-ring that seals the thermo-element. The adjustment was easy with a scanner.