As long as you have a driveway, garden hose, and a few gallons of empty containers to dispose of old coolant, doing the job yourself is the best way to go. You can flush better and for cheaper on your own.
You basically drain all coolant from the rad by disconnecting both hoses (lower first), the top at the t-stat housing, and the bottom at the rad. Remove the overflow tank and empty it, and rinse. Then flush the rad, and engine block with the garden hose until clear water flows out. You leave the drain pan under until it gets clear, then let the clear stuff run on the ground. All that comes out after that are particles. You can back flush by putting the hose on the opposite rad hose, and even flush the heater core separately by removing it's two hoses that are easy to get to as well. Replace the t-stat(and gasket) and rad cap if you'd like. Install all hoses/housings and fill the radiator with pure antifreeze until full, then bleed the system of air. The total capacity is 13 quarts, and I only used pure antifreeze. I added about 6 quarts of pure coolant before it wouldn't take any more, so no need for 50/50 unless you're topping it up after cooling system work.
I want to make a write up of this method eventually, I've flushed 2 3800's this way while converting from dex to green. It's the best method to use. There's literally not an ounce of the old stuff left afterwards, and you will get all loose particles out. If you were really strict on flushing, or had mixed coolant resulting in sludge, fill with water completely and some powder tide to clean the system, and drive like that for a week (in climates above freezing). This will dissolve a lot of the sludge. After a week drain, flush, and refill with pure antifreeze.
