I rechecked my notes and it was actually 62%, not 64%. At about 5 years old the overflow tank was down about 6 ounces. After I had already bought OEM coolant to top up, I learned that I could have just added distilled water and still maintain the 50% concentration. I haven't had any recurring coolant loss problems. When I did a radiator drain and fill, I added less coolant % to get the system down closer to 50%.