I just bought 2 gallons of pure stuff and added it to my cooling system after the flush (engine block full of water). It's a 13-ish quart capacity, and I added 6 quarts of pure antifreeze. That's just about the 50/50 you want.

And flushing with water using a hose is the way to go. Back flushing gets some more crud out and changing the t-stat restores the cooling system's efficiency.