Yup, can't trust it, fluctuations are crazy. What you can do ( what i do too ) is:
-Be aware of the offset between the home scale and the gym scale. Weigh yourself right before gym at home and then right when you go at the gym. That way the differences won't confuse you.
-Don't take the bf/hydration readings to the letter. But use them exactly as you said, quantitative. What i do is always measure under the -relatively- same conditions, morning, before breakfast, after toilet. Take the scale readings, also the waist/height bf measurements. If you have calipers, even better. Put the numbers in an excel file, make xy graphs of them. Those combined with the mirror test can give you a very accurate 'where am I going' feel.