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.
If my gym has calipers, i'll get my bodyfat tested monthly and see how it compares to the scale measurements.
Still want a very accurate bodyweight measurement on that scale though. thats most important. i need to find a balance scale to compare myself to, and then calibrate my scale to it. my gym currently uses a spring scale but i know they have a balance scale stashed away somewhere....
bodyweight will be taken at home completely naked right after my morning piss every time.
edit: it appears that after reading online, digital spring scales like i have offer the most accurate measurements! it's true that i got 198.2lbs about 4-5 times in a row. so yes its consistent. just have to know if its calibrated right.