All you have to do is to mix a small amount of antifreeze half and half with water and test it. See how many balls float. This is what you are aiming for.

Antifreeze is heavier than water, the more balls floating the better.

I actually mixed the antifreeze in a 1954 Ford without opening a drain that I was leery of. On a cold day, I sucked as much water out of the radiator as I could, then I poured antifreeze in slowly. When I sucked more out of the radiator, it was almost pure water. After several repetitions, I was able to get the proper amount of antifreeze in. I ran the car and tested the coolant 50-50 on the nose!
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3 8s & 2 7s 725,000 miles total