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The quality of your tap water makes a difference here.
I'm pretty careful with fluids, but I'm still fine with flushing using water from a garden hose. I drain the radiator, remove the thermostat, and open the heater core valve. The upper radiator hose is removed from the radiator and redirected into a bucket. I put the hose into the radiator, turn the water on, and start the car for a few seconds.
I do capture the first few drains completely, and pour them down the sanitary sewer. (Not a septic tank or storm sewer.) After that, any remaining antifreeze is so diluted that it poses no danger so I'm not concerned if some of the flush water ends up running down the gutter.
If I have a spare gallon of distilled water, I'll use it for the final flush. I pour the distilled water through the engine (off!) using the upper radiator hose, letting it drain out the radiator petcock.
I fill with the correct amount of full strength antifreeze first, then top off with the distilled water. This way you don't have to be concerned with how much water is left in the engine block or heater core.