I ran into a problem somewhere in my gravity readings during my brew today. I took a pre-boil gravity reading of 1.036 which was right on target what what I expected it to be. The pre-boil volume was 6.75 gallons. My post boil gravity reading was 1.037, which simply can't be correct with a volume of 5.5 gallons. I was expecting a post boil gravity of approximately 1.043. I took these gravity readings with a refractometer (Brix) and made the conversions (double checked).
How can I calculate my post boil gravity mathematically based on my pre-boil gravity and the two volumes?