no calculator needed... you shoot the marks with the decided load and bullet. From the actual shooting then create a paper drop chart and tape it to your stock and use that for reference. long distance shooters do it all the time... although I am not one of them.

To tell you the truth though, I do the same thing for Rondies using a Lyman peep site or an adjustable Creedmore tang sight.

Hmmm..

http://www.mil-dot.com/user-guide