Remove all water from the block

If you used distilled water, I'd just buy me a couple gallons of straight undiluted anti freeze and mix with the distilled water to get the correct specific gravity for it. Wouldn't worry about draining the system completely. You'll have to have enough coolant in it so you can run it and circulate it to check the mix. Also as a side note I'd pull the thermostat and fill up to that point thru the block to help prevent air pockets. Then install the thermostat and go from there.