This is able to be calculated bc the total gravity of your wort stays constant through the boil. Therefore, V1*gravity1=V2*gravity2.
In your case, V1 is 6.2 gallons, gravity1 is 74 (use the numbers after 1.--- in your read specific gravity) and V2 is 5.4. Solve for gravity2 = 85. Therefore your OG post boil is 1.085