Area of cricle- Radius squared x pi. So, 3x3=9xpi(3.14)=28.26 cu ft per foot of bag. Using your factor of .488(I have no idea if that is a good factor to use) would be 13.79 bushel.

This will be the absolute max attainable, bags do not fill in a perfect circle. It is also very difficult to get an accurate emasurement of bag length, I usually try to eyeball about 1/2 way down on the wedge at the end.

Theres lots of places for inaccuracies to creep in. Did every batch of feed took out get recorded? "Dirtier" corn with more cobs and trash will weigh much less. 

Edit-I guess I had a brainfart, Blanes numbers are much better than mine.