The best way to reduce error from weight fluctuations is to weigh yourself either every day or several times per week and take the average. Doing this will wash out minor variations in weight caused by fluid retention.
If you wish to weigh yourself every day, or even just a few times per week, to get an average weight for the week, turn on Log Daily Weights in Advanced Features. Once this feature is on, hit the “Log Daily Weight” button on your app’s dashboard to input that day’s weight. The average of these daily weights will automatically be imported into your check-in.
To learn the possible causes of weight fluctuation, read here.