The bags are always off. I go off the bottle that I feed from.
I had a nutrition scale, set it to ounces instead of grams, and pour it from my pump into a bottle while on the scale and zeroed out. It gives me an accurate measurement of what each side is producing per pump/when I poured it into storage bags.
Never use the measurements on the bags. They are not accurate in the slightest. I pour my milk from my handsfree pump into a bottle for accurate measurements before pouring it into the bag.