Is your temp gun calibrated? Honestly I think it sounds like you’re making it over complicated. Either way, mix up a glass of ice water for half a minute or so and check it with your temp gun. Should read 32°. If it doesn’t then it’s not accurate.
Also some fluctuation is natural. As the probe reads the set temp or reaches it, it reduces or stops supplying power to the rhp. Unless you have something like a piece of slate under the panel that will retain heat for an extended period you will necessarily see a fluctuation in temps
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