The truth is, with modern vehicles, there is no way to get 100% of the old coolant out. That went away with the appearance of pressurized coolant reservoirs and the advent of capless radiators and quick disconnect hoses. The old coolant flush machines were connected inline on the upper radiator hose and run in bypass mode until the thermostat opened, then we'd flip a switch and the system would catch the old coolant being pumped to the machine while pushing fresh coolant in. Back then we got as close to 100% new coolant as you could possibly get without pulling the engine.
Now? We use a machine with a single hose that pulls a vacuum on the reservoir to collapse the hoses and suck out all the coolant it can with the engine running, then we flip a switch and force fresh coolant into the system, and repeat that a few times. The end result is less than 100%, but it's as good as can be done now.
I can tell you from experience that a drain and fill is fine on a 3.5 so long as you burp it correctly after (Haynes has the proper procedure in their manual), as that was all I did when I did my mother's water pump on her 3.5L Taurus in their driveway.