Only after replacing most of the cooling system did I buy an infrared thermometer and realize that my water temp gauge is reading about 50 degrees too hot!

I've replaced the water temp sender (twice), so either the gauge itself is not functioning properly or the resistor between the sender and the gauge has been removed. I can't find the resistor under the dash, so I think it may be missing. Does anyone know the resistance of that resistor? I'm hoping I can just put the right resistor in there and it will read the correct temperature... Does that sound likely?
Thanks!