You can get a very good little digital meter at Sears for $30 with a Duty Cycle range. Then you can leave your calculator out of it. You will also have a great little compact meter that you can throw in your toolbox.

I learned a long time ago that your margin for error is drastically reduced by using the correct test instrument for the job rather than extrapolating the reading. I reserve that for when I'm in a pinch and can't make the measurement any other possible way.

Good luck,