I just guessed, then check. If it is too high, I add some water into another 5g bucket, and then mix the two together until it's at 1.009. Also, I hope your not making a QT tank full of new water, I'd add some water from your display and RO/DI freshwater as well, so it isn't that big of a shock. That way you don't need a whole lot of 1.009 water.