Reading the temperature gauge

RedFish said

The 3 little dots at bottom of screen are the factory calibration targets. I could go to the book and report what temp that middle dot ( 23 ohms ) represents. Those targets cant be seen once the gauge is installed though.
If it helps, when the gauge was new, the first heavy hash mark ( shown as "pretty good" above ) is 34 ohms, approx' 180 degrees, and where OEM thermostats begin to function.

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Thanks for the info. It prompts a few more questions. If the center dot equals 23 ohms and the first heavy mark is 34 ohms ~180º, then what are the temp and ohm values of the other two dots for a new gauge , per the book? In your experience, as the systems age, do they tend to read cooler or hotter than they did when they were new? How do I test the temperature sensor and what sort of resistance value should be expected? (i.e. How do I know if the temp sensor is good?) Is there a difference between a senor for an idiot light and a gauge? How can I tell which one is which?