From an ex benchrest shooter:
Identify the edges of your group that are furthest apart and draw a line through the centre of the opposing bullet holes. Set your Vernier on the group so that the edges of the blade form a 90 degree angle with the scribed line at the furthermost edge and take a reading.
Record this reading and subtract your bullet diameter, this will give you your C-C group.
If you want to, divide by 1.09 to get MOA.
Hope the pix will help explain.
My 243 Group.
"I meant to shoot the pike but the duck got in the way"