I have a 2006 GC with 3.8. It has a rear heater. I plan to put in a new radiator next weekend. Before I do that, I plan to run Prestone Flush and Cleaner for one day (about a 10 hours, I am a delivery guy) on Friday before the work. Mainly for the front heater core. Seems it took longer than it should have to warm up this past winter. So I will run that, then the weekend, I will drain the radiator, then refill with distilled water and run for several minutes, then drain again. Then I will replace the upper and lower hoses and the radiator. My question is do I have to be concerned about this flush not draining sufficiently from the entire system? How will I get a proper 50/50 mix if I don't know if the system is actually drained? Do you have to check your fluid after it is mostly filled with a gauge instead of relying on a 50/50 mix?