How much coolant for 512TR flush?

I've always had someone else change the coolant in my 512TR and am going to do it myself this time. The manual says 20 litres is the capacity (over 5 gallons!). Does anyone know if a drain & fill uses all of that? If so, I'll need to get 3 gallons of coolant for a 50/50 mix. I know you also have to fill it and run it until the thermostats open to fill the radiators, then bleed it. Anyone have any suggestions here? Thx!