If you are doing a perimeter and measuring the acres from the perimeter, that is the correct acres in the field.  If FSA says something different then that, they are wrong.  That's what you get when you try to measure fields by looking at a picture taken from four miles away. 

The "applied acres" you show when you finish the field are virtually sure to be greater then the actual acres measured by drivign the perimeter of the field.  How much greater indicates how much you double-applied in the field.  

For field operations at tractor speeds I have always run the lights at the smallest setting.  I think 4 or 6 inches per light on the equipment I have used.  Running at 20 mph you might want it a little looser.  Running at 100 mph I imagine you would want it a lot looser.   My first lightbar the default setting was 5 feet per light.....it was originally intended for aerial use.