There are two ways to improve what you are doing to flush your cooling
system and refresh your coolant.
The best is to locate the drain plug on the engine block and drain all
the coolant. Then you just get the specification from your manual for
total capacity of the system and add appropriate amounts of water and
100% anti freeze to create the ratio of mix you want. Up to 70% gives
benefit for anti freeze and anti boil, but 50% does a better job of
heat transfer because water has what physicists refer to as a higher
"cooefficient of heat." Water transfers heat better.
The other method is open the drain plug on the radiator and with the
motor running, run a hose into the water intake and let water flow
through the system until it runs nearly clear. Then you have all
water (almost) in the system. Shut off the motor, shut off the water,
let it drain. Then, get the capacity specification as in method
one. Close the drain plug. Start filling with anti freeze and if
your system has an 8 liter capacity, and, for example, if you want 50%
solution, add 4 liters of anti freeze and then fill the rest of the
way with water. This method works pretty well, but draining the block
is more certain and takes less time.