I think it may be a simple means to approximate the fourth root of 26 which is 2.25. The standard deviation is the square root of the sum of the positive differences. SQRT(549) = 23.4. There is obviously nothing wrong with his chronograph if his adjusted MV to simulate 5 shots through analytical statistics is going to be only +2.2 fps for the middle load. Keep in mind he is trying to keep it simple. Run the same thing through Excel's regression analysis tool and you get a standard error of 2.75. Close enough. I still don't understand the 1/12 part though.
