If you try to premix it before adding it to the reservoir you're not ending up with the concentration you hoped for after it circulates through the system. The water already in the system will dilute what you're adding.
I always add pure antifreeze into the reservoir after a flush. I know that if the system has a 9 quart capacity and I manage to just add 4 quarts of antifreeze I'll have about a 44% mixture after everything blends together.
