Problem with pre-mix is.....after you flush a system with water, some is left behind. Now how are you going to get the concentration up to 50/50? It will always be less.
Easy. Just get one of these refractomers which is calibrated for antifreeze concentration by percentage points. I have one. Very easy to determine the exact percentage of antifreeze in the coolant, has scales for both antifreeze types.
SMARTSMITH Antifreeze Refractometer Displaying in Fahrenheit for Checking Freezing Point of Automobile Antifreeze Systems and Battery Fluid Condition. Battery Acid, Glycol, Coolant, Antifreeze Tester
https://a.co/d/eujZbKI