From what I gather, there's all kinds of ways to have field loss but here's what my state website has to say..... 

Convert the number of kernels in 10 square feet to bushels per acre by dividing by 20. 

Table 7. Corn Loss Measurement

Here's the web page: http://www.aragriculture.org/crops/corn/harvesting.htm

I think they got this from Iowa.

Hope this helps!

Jen