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Alot of eggcelent advise listed above for sure. But let me echo Max a bit. Dont be so concerned about hitting 250 exactly, All eggs are different but also all fires are different. So much depends on atmospheric conditions, etc. When she starts to settle in somewhere in the mid 200's, just go with it. Having a stable temp is more important than what the exact temp is. I have cooked low/slow anywhere from 200 to 300.