50:50.
I use Mercedes Blue 1.5 litre bottle to 1.5 litre distilled water.
Make sure all suspect hoses are replaced, leaks including waterpump seal are pressure tested, radiator cap is functional and holding pressure and if there is any doubt get the radiator cleaned out by rodding.
Flush out all old coolant including heater core with several flushes of clean water. Find out what the capacity of the cooling system of your car is supposed to be. Put in half that volume of Mercedes "Blue" coolant and then top up with distilled water and forget about it for several years.

Here is a picture of the total amount of M-B Blue needed for a 6.9 - not including all the distilled water I used for the flushing as in summer time my property runs on bore water and that wasn't going into my car
Bill