When measuring something that varies, like weight, it's best to take multiple measurements per day and then average. During my geeky phase, I'd do three a day (morning, mid-day, and night), dump them into Excel and compute a daily average. Then my weekly average was the average of the 7 days' averages.
Do this, and you'll find that your weight varies much more within days than the averages do between days. So, you won't worry or sweat the variation on any given day.
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