How do I make sure I have 50/50 mixture in the system?
It doesn't have to be "spot b0llock" with regards the mix.
I followed the how-to (applying a bit of my own common sense as well), and I ended up going through 11 litres of anti-freeze by the time the job was done.
I did 3 lots of drains & fresh water refills, then it was 2 lots of drains & refills of a 50/50 antifreeze/water mix, followed by a final refill of what was probably a 60/40 mix of antifreeze/water.
I made the final mix that little bit stronger to allow for any water that may still be in the system, as I was only losing around 5 litres of coolant every time the radiator was drained.
The final 1 litre (800ml to be precise) went in the next day, as the fluid level had dropped slightly when I checked the expansion bottle the following morning.
The final 800ml was not diluted down either, it went in neat.