A person just runs things as slow as possible and yet get the job done.

Stripper plates are brought in as close as possible, but still letting the stalk feed through.

If you want to measure how much shelling loss in on the ground from your head you need to combine at the normal operating speed.  Stop the combine quickly while shutting the combine down.  Back up 10-20 feet.  Get out and search for kernels shelled off by the head, but yet before the back of the combine has put out any trash/kernels. 

Now if you have, for example a 8 row head in 30 inch rows, 8 rows wide by 26 inches long equals 1/1000 of an acre. 

Here is the math. 43560 sq ft per acre.  43560/20 = 2178    2178 / 1,000 = 2.178 ft      2.178 x 12 = 26.136 inches.

So 20 ft wide x 26 inches = 1/1,000 of an acre.

(A 6 row 30 inch head would be 6 rows wide x 35 inches long = 1/1000 of an acre.)   

You take a count and find, say 45 kernels in this 20 ft wide (8 rows) x 26 inch long area.  45 x 1000 = 45,000 kernels/ acre loss. 

 45,000 / 90,000 = 1/2 bu loss per acre from shelling at the head.