I am about to make up some of your classic car coolant at recommended 12%.

My car holds 10 litres.

I have two of your 5 litre demin water containers. How much concentrate do I add to get 12% mix?

I realize I will now have over 10 litres total (possibly 11 or 11.5) and that is where I am going crazy. Or am I better to drain a specific amount out of one container then add the concentrate?

Sorry for sounding like a dummy I just can't work it out.