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"How many minutes do you all wait after each vent adjustment before you declare the egg stable?"
For some folks here, the answer to that question is "60". I'm not that patient but you should know that some are.
I think the only mistake you made was trying to be too specific on your temp. With a little time you will learn APPROXIMATELY what settings will get you APPROXIMATELY what temps and you will get comfortable with it. For example, I know that if my bottom vent is open about 1/4 inch and the daisy wheel is set so that each of the 5 openings is barely open, my XL will settle in at a temp somewhere between 220 and 250.
The difference between cooking at 225 and as high as 275 isn't really worth discussing as long as you have a way to monitor the temp of what you are cooking. When it reaches the desired temperature, take it off.
You will probably still find yourself chasing temps from time to time. When that happens just stay close to the egg until you are happy with it. In that setting, the only excuse for leaving the egg is if you run out of beer. A fridge or cooler near the egg can solve that problem.