Why worry about so? Let the group size on the target tell you. Just because it is easy to calculate doesn't mean it tells you anything.
I think Muledeer's point about chronograph shays to do with the fact that the shorter the distance between the screens the more likely that small measurement errors will be multiplied. That's why the Oehler 43 has three sensors and compares the reading from 1-2 to 2-3. Don't they cal that the proof channel? Hard to remember after a mound of turkey, several fingers of brandy, and the packer to under review!
