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Plus 1 for 3" is plenty and surface temperature.
It takes as little time for the temperature in the center to get to where you want it to be but in the meantime, the temperature on the outside is still being subjected to the heat source and getting above your desired temperature. My study between the temperature on the glass surface and the temperature 12" into the center of the carboy never saw a difference greater then 4 °F when using a heat belt applying heat evenly. Crazy temperature differences in fermentation tanks does happen but they are not that much on the homebrew level when you're doing 5 gallons at a time....RDWHAHB
A few years ago I did the same test and saw a similar max variance at high kreuzen.