There is no general relationship. Sometimes it will be higher and sometimes lower, depending on whether your heating element is turned off or on at the moment and how long it's been so. Also it depends on the structure of your incubator (where the heating element is, where the fan blows, where the wiggler is)
The point of the wiggler is to tell you, as best as you can, what temperature the inside of the eggs is likely experiencing. That is the ONLY TEMPERATURE THAT MATTERS. Air temp can do what it likes as long as it produces a fairly constant 99.5ish F *inside* the eggs
Any tips on making a false egg. In other words what to fill the wiggler with. I want it to replicate the egg as well as possible, I have seen everyting from water to hair gel mentioned. I want a make a couple of permanent eggmometers and was actually thinking of making them with silicon and plastic eggs with an aquarium thermometer stuck in it permanently.