In 50 years in photography, every temperature I've ever used was a "drift-by". All temperatures are fleeting, with the best of processes and procedures. Depending on this "averaging " method as your process is not acceptable, imo. A color print starts to go warm toned when you get a degree high, and bluish a degree too low. Without an elaborate and pricey temp set-up, you must rely on cookpots, stoves, and refrigerators. But it does work, although it might take an hour of prep and dry-runs for a single print. But you have to get it right. 1/2 of a degree , is your "drift-by". And it's best to do your prep and dry-runs to where even that 1/2 doesn't show up on the thermometer.