It is extremely hard, from my experience, to get two accurate POIs, which is what you need to determine the BC from them.... Even a tiny error in measuring the center of the group will give a fairly large error in the BC.... In addition, you need to correct for your atmospheric conditions.... The biggest change will be altitude, if you are not at sea level.... Make sure you use the Environmental Applet to input your altitude, barometric pressure, temperature and humidity.... We are at 2500 ft., and if I measure the BC as 0.05, it ends up at 0.04 corrected to sea level, as it should be....
This is not to say that your BC is not 0.04, that is within the range of values I would expect for that pellet.... just that you need to be careful to de everything correctly, it's easy to forget to correct for altitude, for example....
Bob