Date: 1/3/2007 10:59:57 PM
Author: Garry H (Cut Nut)
It is a comparitive figure for the surface area of a stone compared to a Tolkowsky round with a thin girdle.
+1% = a spread that would be 0.01ct bigger for a 1ct size
-15% is very small - and typical for a princess with an appropriate safe girdle thickness for example.