I just drain the coolant then flush through, then put 2 1/2 Litres of anti freeze in, I always run a WYNNS cooling system cleaner for a few hundred miles first, gets all the crap out, but becomnes a congealed mass of jelly in the expansion tank, so this needs cleaning out also, but very good stuff, they say idle for 10 minutes, always done a few hundred never had a problem, has helped heater output on most cars I have used it on.
tabetha