I've thought about this too, but heres my conclusion. Cost of BAC water is way way lower than the cost of GH. So use as much as you need, to get all of that precious GH out of the vial. You could probably use even less water, but the less water you use, the more exact the measurement gets. With a limited amount of BAC water, the more you are likly to leave behind in the vial, as you try to get that last little bit out. There is always a tiny bit that won't draw up into the needle and if it's very concentrated, that might mean losing .25iu or even .5iu, in just a few drops. I don't now about you, but I find that idea unacceptable! haha
But sure, if you were running short on water, you could always use less. I've gone as low as .5ml of water for a vial, Personally, I wouldnt make a habit of it though