The formula is X + 2Y + 1 inch
X = length of quilt top (normal border length)
Y = width of border
1 inch = 1 inch
You need the normal length for the border plus twice the width of the border plus one inch.
A 60 inch quilt with an 8 inch border would need a minumum of:
60 + 8 + 8 + 1 = 77