Yes Djay, I worked out when doing all the flushes that I could only refill through the header tank to a maximum of 4 3/4 Lt with the thermostat closed so when I was satisfied that the whole system was full of clean water I drained down the system ( as much as I could) and refilled with neat concentrate at just under 5 Lt. The last 1/4 Lt I mixed in the bottle 50/50 with water. I then started the car and took it to thermostat open temp where after cooling I was able to top the system to the correct level on the header tank using the remaining mix.. After a couple of days of running I then checked the coolant concentration and it was spot on for a 50/50 mix strength.
So the final fluid used in the system consisted of just under 5 Lt of concentrate and 5Lt of distilled water. This was effectively mixed by the water pump within the engine when running and allowed me to obtain the correct mixture strength.