Now load that with a powder measure that varies +/- 0.4gr with Varget (meaning dropped charges span a range of 0.8gr)
Wow. If your powder measure is really that bad, we ought to chip in and treat you to a new one that is at least mediocre.
With Varget, my Lee powder measure has a standard deviation of .11 grains, measured. So 95% of charges will fall between plus and minus .22 grains, and the math works out as shown.
.....