Also, I do believe you can add 1 or 2 bottles of prestone super radiator flush to the premixed coolant, and drive it like that for a bit and allow it to break down more sediment, then waterflush it about 4-5 times.
You may also want to back flush the radiator after driving with the superflush by removing both radiator hoses, radiator cap on, and running as much water possible through the bottom outlet. I once had a spare radiator hose, a pvc reducer and a spare length of garden hose setup together so the hose would screw onto the end of the normally used hose or sprayer nozzle, and the other end would clamp onto the radiator. Worked especially well when the radiator was removed and was set on its side, inlet down, gravity on the assist for removing built up sediment. It actually worked quite well for me.